Blue Large-Amplitude Pulsators and Other Short-Period Variable Stars in OGLE-
IV Fields of the Outer Galactic Bulge
J. B o r o w i c z1, P. P i e t r u k o w i c z1, J. S k o w r o n1, I. S o s
z y ń s k i1,
A. U d a l s k i1, M. K. S z y m a ń s k i1, K. U l a c z y k2,
R. P o l e s k i1, S. K o z ł o w s k i1, P. M r ó z1,
D. M. S k o w r o n1, K. R y b i c k i1,3, P. I w a n e k1,
M. W r o n a1, M. G r o m a d z k i1, and M. J. M r ó z1
1 Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478
Warszawa, Poland

2 Department of Physics, University of Warwick, Coventry CV4 7AL, UK

3 Department of Particle Physics and Astrophysics, Weizmann Institute of
Science, Rehovot 76100, Israel

ABSTRACT

In this work, we search the OGLE-IV outer Galactic bulge fields for short-
period variable objects. The investigation focuses on unexplored timescales
roughly below one hour in an area containing about 700 million stellar sources
down to $I\approx 20$ mag. We concentrate mainly on Blue Large-Amplitude
Pulsators (BLAPs), which represent a recently discovered enigmatic class of
short-period hot subluminous stars. We find 33 BLAPs in the period range from
7.5 to 66.5 min. Thirty-one of them are new discoveries, which increases the
number of known stars of this class to over one hundred. Additional eighteen
objects with pulsation-like light curve shapes and periods ranging from 17.3
to 53.7 min are presented. Very likely, these are $\delta$ Sct/SX Phe-type
stars, but some of them could be pulsating hot subdwarfs or BLAPs. We also
report on the detection of five eclipsing binary systems with orbital periods
between 61.2 and 121.9 min and three mysterious variable objects with I-band
amplitudes larger than 0.9 mag. A spectroscopic follow-up would help in the
final classification of the variables.

Stars: oscillations (including pulsations) — Stars: variables: Blue Large-
Amplitude Pulsators — binaries: eclipsing

##  1 Introduction

Short-period variable stars constitute a broad and diverse sort of celestial
objects exhibiting brightness variations on timescales below about one hour.
Among them could be single pulsating stars of early spectral types as well as
close binary systems formed of low-mass red dwarfs or brown dwarfs. In recent
years, the number of known short-period variables has significantly increased
thanks to large-scale photometric surveys. One of the latest discovered class
of pulsating stars, known as Blue Large-Amplitude Pulsators (BLAPs), was
identified using data from the Optical Gravitational Lensing Experiment (OGLE;
Udalski et al. 2015). The first BLAP was found in the Galactic disk at
longitude $l=288\makebox[0.0pt][l]{.}^{\circ}06$, while other thirteen objects
were detected toward the Galactic bulge (Pietrukowicz et al. 2017). The
original 14 pulsators exhibit a uniform class of periodic variables with
pulsation periods ranging from about 20 to 40 min and I-band amplitudes
between about 0.2 and 0.4 mag. The stars were recognizable by their
characteristic sawtooth-like light curves. Spectral analysis indicated that
BLAPs belong to hot evolved objects with effective temperatures around 30 000
K. However, subsequent studies suggested that BLAPs represent a more diverse
group. Four objects characterized by higher surface gravity and shorter
periods (below 8 min) were discovered in data from the Zwicky Transient
Facility (ZTF) survey (Kupfer et al. 2019). Further discoveries (McWhirter and
Lam, 2022) were made using combined data from ZTF (Bellm et al. 2019) and Gaia
space mission (Gaia Collaboration et al. 2021). Three new objects were
detected in the OmegaWhite (OW) survey (Ramsay et al. 2022). The OGLE data for
the Galactic disk revealed 20 new BLAPs with periods ranging from 8 to 62 min
(Borowicz et al. 2023), while data for the inner Galactic bulge allowed the
identification of additional 23 objects with periods ranging from 14 to 74
min, some of which showing an extra bump in the light curve (Pietrukowicz et
al. 2024). Very recently, a BLAP located on the outskirts of the bulge was
detected (Chang et al. 2024) using data from the SkyMapper Southern Sky Survey
(Onken et al. 2019). So far, the number of known BLAPs stands at approximately
80, with over 60 of them identified by the OGLE survey.

Precise boundaries of parameters characterizing BLAPs remain uncertain. Recent
studies suggest that the variables can exhibit a wider range of periods and
amplitudes than previously thought, including multi-periodicity (Pietrukowicz
et al. 2024, Koen et al. 2024). Until recently no BLAPs in binary systems were
known. Using Transisting Exoplanetary Survey Satelite (TESS) photometry,
Pigulski et al. (2022) detected a pulsator with a period of 32.37 min orbiting
a hot main-sequence (MS) star with a 23.08 d period. Another object, TMTS-
BLAP-1, discovered by the Tsinghua University – Ma Huateng Telescopes for
Survey, likely constitutes a wide binary system with an orbital period of 1576
d (Lin et al. 2023a).

BLAPs, as a class of variable stars, have sparked several hypotheses regarding
their nature and evolutionary pathways. One of the proposed hypotheses
suggests that BLAPs represent low-mass stars in the pre-white dwarf (pre-WD)
phase, where they generate energy through residual hydrogen burning atop a
degenerate helium core (Romero et al. 2018). Alternative models propose that
BLAPs may be stars undergoing helium burning in their cores (Byrne and
Jeffery, 2018; Wu and Li, 2018), or they could be shell helium-burning
subdwarfs (Xiong et al. 2022). Stability of the BLAP pulsation periods,
characterized by a change rate of $|\dot{P}/P|<10^{-6}$ yr-1, indicates that
these objects evolve on nuclear timescales. Proposed mechanisms explaining the
evolutionary trajectory of BLAPs include origin from an evolved binary system,
following a common envelope phase with Roche lobe overflow (Byrne et al.
2021), or a result of a helium WD+MS merger (Zhang et al. 2023). Additionally,
it has been proposed that BLAPs could emerge as surviving companions of
single-degenerate Type Ia supernovae (Meng et al. 2020). The study of BLAPs
can give us an insight into the structure and physical processes in hot
pulsators—stars that oscillate due to the presence of a Z-bump in the oppacity
at temperatures of about 200 000 K (Byrne and Jeffery, 2018).

Among the short-period variables, binary systems form a separate class of
objects. While the vast majority of binary stars have orbital periods longer
than 0.2 d, there are also systems with shorter periods. In AM CVn systems, a
type of cataclysmic variable stars, hydrogen-poor matter from a compact
companion is accreted by a white dwarf. Binary systems with orbital periods
below 30 min are usually formed of two white dwarfs. In recent years, the
detection of multiple compact binary systems has become possible thanks to
large-scale and all-sky variability surveys like ZTF, TESS, and Gaia (e.g.,
Burdge et al. 2019, Burdge et al. 2020, Barlow et al. 2022, Ren et al. 2023).

In this work, we utilize time-series data collected by the OGLE project for
fields located in the outer Galactic bulge. These fields have been surveyed
for variable stars such as Cepheids (Udalski et al. 2018), RR Lyrae-type stars
(Soszyński et al. 2019), $\delta$ Sct-type stars (Soszyński et al. 2021), and
Mira stars (Iwanek et al. 2022). Here, we search for objects exhibiting
periodic variations on timescales shorter than one hour. This paper is divided
into several sections. In Section 2, we give a brief overview of the OGLE
survey and the data we collected. Section 3 presents the data analysis
methods. The detected short-period variables are described in Section 4. We
wrap up with a summary of our results in Section 5.

##  2 Observations

Our variability search utilizes the photometric data from the fourth phase of
the OGLE project (OGLE-IV), which commenced in March 2010. OGLE has been a
leading large-scale survey for over three decades. The project observes dense
stellar regions of the southern sky with the 1.3-meter Warsaw telescope
situated at the Las Campanas Observatory, Chile. The observatory is operated
by the Carnegie Institution for Science. The primary regions monitored by OGLE
are the inner Galactic bulge and the Magellanic Clouds. Additional shallow
observations of the Galactic disk and outer Galactic bulge are conducted since
2013 as the Galaxy Variability Survey (GVS), with a cadence of a few days over
the first 100 epochs. Currently, the project covers an area of nearly 3600
square degrees. OGLE-IV encompasses 622 Galactic bulge (BLG) fields, 444 of
which belong to the outer bulge and are distributed approximately evenly
around the Galactic center. The OGLE-IV BLG fields cover an area spanning
roughly from $-15^{\circ}$ to $+20^{\circ}$ in Galactic longitude and from
$-15^{\circ}$ to $+15^{\circ}$ in Galactic latitude. Coordinates of the OGLE-
IV BLG fields can be found at

https://ogle.astrouw.edu.pl/sky/ogle4-BLG/

The OGLE-IV project employs a 268.8-megapixel mosaic camera with a pixel size
of $0\makebox[0.0pt][l]{.}{{}^{\prime\prime}}26$ and a 1.4-square-degree field
of view, consisting of 32 single 2K$\times$4K CCD detectors. Observations are
conducted using I and V filters, which closely resemble those of the standard
Johnson-Cousins system. Observations in the I-band are much more frequent than
in the V-band. Exposure times in the I-band are of 25 s for the GVS fields and
of 100 s for the inner bulge fields. Unfortunately, in the case of the outer
bulge fields, regular I-band measurements (ranging from 90 to 250 epochs) are
not supplemented with V-band measurements. The I-band brightness range in GVS
fields spans from about 10.5 to 21 mag. An exception are seven fields, from
BLG705 to BLG711, covering the central part of the Sagittarius dwarf
spheroidal galaxy (SgrDG). The data for this small region were taken in both V
and I bands with 150-s exposure times in years 2011–2014 and contain around
150 epochs in the I-band. The OGLE-IV observations were stopped on March 18,
2020, as a result of the COVID-19 pandemic, but regular monitoring resumed on
August 12, 2022. Technical details on the survey can be found in Udalski et
al. (2015).

##  3 Data Analysis

The search for short-period variable stars was conducted using time-series
data in the I-band. We analyzed 444 OGLE-IV fields for the outer Galactic
bulge area from the shallow survey and seven fields from the deep survey of
the SgrDG, totalling about 700 million stellar sources. We focused on sources
with more than 50 measurements and brighter than $I=20$ mag. Our initial step
involved an identification of periodic signals for all the sources using the
FNPEAKS111http://helas.astro.uni.wroc.pl/deliverables.php?active=fnpeaks&lang=en
code, which employs a discrete Fourier transform on unevenly spaced data. We
explored a frequency range of $0~{}<~{}f~{}<~{}500$ cycles per day
($P~{}>~{}2.88$ min). Subsequently, only objects exhibiting a significant
signal-to-noise ratio were retained. The variability signal cutoff varied
across different fields, mainly depending on the number of observations
($S/N\sim\sqrt{N_{\rm obs}}$).

In the following stage, all variable objects with periods shorter than 0.05 d
underwent a visual inspection. We refined our candidate variable lists by
eliminating artifacts (objects displaying spurious variability). Any
noticeable outliers in the light curves of genuine variables were removed for
further analysis. About three hundred variable stars with periods below 60 min
were found. Stars exhibiting amplitudes exceeding 0.05 mag underwent further
investigation. BLAP candidates were identified based on the characteristic
light curve morphology. Variables with low amplitudes and sinusoidal-like
light curves could not be classified solely based on the photometric data.
Most of these objects are likely low-amplitude $\delta$ Sct stars, similar to
those found in several recent studies (e.g., Pietrukowicz et al. 2022, Lin et
al. 2023b), although among them there can be other short-period variables such
as pulsating subdwarfs, SX Phe-type stars, or low-inclination eclipsing
binaries. Light curves exhibiting uneven minima upon doubling of the period
were also selected for further inspection. Instrumental magnitudes were then
converted to the standard photometric system. The accuracy of the calibration,
which follows the method outlined in Udalski et al. (2015), achieves a
precision of 0.02 mag within the Johnson-Cousins system.

As there were no available V-band measurements in the examined outer bulge
fields, we constructed color-magnitude diagrams (CMDs) using data from Gaia
Data Release 3 (Gaia Collaboration et al. 2021). The diagrams were prepared
based on the mean G-band magnitudes and $(BP-RP)$ colors for stars located
within a $0.1\times 0.1$ square degrees region. For two objects without
available $(BP-RP)$ measurements, data from the DECaPS DR2 (Saydjari et al.
2023) were used. To analyze the short-period variables, we transformed the
brightness measurement timestamps from Heliocentric Julian Date (HJD) to the
more precise Barycentric Julian Date (BJDTDB). The previously found periods
were then corrected using the TATRY code (Schwarzenberg-Czerny 1996) which
utilizes multi-harmonic functions. The final list of BLAPs was compiled based
on the morphology of the light curve, period, amplitude, and position on the
CMD. All objects of interest were then examined for the presence of any
additional periodicities.

##  4 Results

The primary result of this work are the compilations of BLAPs, eclipsing
binary systems, and other (unclassified) short-period variable objects
detected within the OGLE-IV outer Galactic bulge area. Phase-folded light
curves and CMDs for the discovered variables are presented in this paper,
while time-series data and finding charts for BLAPs can be downloaded from the
on-line version of the OGLE Collection of Variable Stars (OCVS) at

https://www.astrouw.edu.pl/ogle/ogle4/OCVS/

For BLAPs, we adopt the same naming convention as for other variable stars in
the OGLE collections. We start numbering the new objects as OGLE-BLAP-NNN from
NNN=062 and continue with the increasing right ascension.

###  4.1 BLAPs

Our search led to the detection of 33 genuine BLAPs. Thirty-one of these
objects are new discoveries. Observational parameters including coordinates,
pulsation periods, magnitudes, and I-band amplitudes of the identified BLAPs
are provided in Table 1. Parallaxes ($\varpi$), G-band magnitudes, and $(BP-
RP)$ colors are sourced from the Gaia EDR3 catalog (Gaia Collaboration et al.
2021). Two of the detected stars were previously identified as BLAPs, namely
OGLE-BLAP-086 as object OW J1826-2812 by Ramsay et al. (2022) and OGLE-
BLAP-093 as SMSS J184506.82-300804.7 by Chang et al. (2024). Positions on the
sky of BLAPs detected in our search and reported earlier (Pietrukowicz et al.
2017, 2024; Borowicz et al. 2023) are shown in Fig. 1. In the bulge area, all
pulsators but object OGLE-BLAP-093 lie within latitudes
$2^{\circ}<|b|<8^{\circ}$. The lack of stars close to the Galactic plane is
very likely caused by high extinction.

Fig. 1. Positions of BLAPs detected by the OGLE-IV survey in the Galactic
bulge area (panel A) and the whole Milky Way stripe monitored from Las
Campanas Observatory (panel B). The inner (deep) bulge fields are shown in
gray, whereas the outer (shallow) bulge fields are in pink. Special (deep)
fields covering the central part of the Sagittarius dwarf spheroidal galaxy
are marked in red. The OGLE-IV Galactic disk fields are depicted in white. All
BLAPs but one object lie within the galactic latitudes $|b|<8^{\circ}$. The
lack of BLAPs at latitudes $|b|<2^{\circ}$ in the bulge area is likely due to
high extinction.

In Fig. 2, we present the period vs. I-band amplitude diagram, depicting the
positions of all BLAPs identified by the OGLE survey, including stars detected
in this study. Additionally, the figure includes single-mode $\delta$ Sct-type
stars from the OCVS (Soszyński et al. 2021). Notably, there are no obvious
differences in the location on the diagram between objects observed in the
disk and bulge fields. All BLAPs with periods shorter than 70 min have the
amplitudes larger than 0.1 mag. The only exception is object OGLE-BLAP-022
with a period of 74.05 min (the longest known so far) and an amplitude of
0.087 mag in the I-band (Pietrukowicz et al. 2024). The highest measured
amplitude in I is 0.428 mag in the case of OGLE-BLAP-078. It can be noted that
the upper bound on the amplitude decreases with the increasing pulsation
period.

Four out of the 33 identified BLAPs have been classified as variable objects
in the Gaia DR3, namely OGLE-BLAP-069, 071, 091, and 094. These objects have
been assigned to the DSCT$|$GDOR$|$SXPHE class. Phase-folded I-band light
curves and I vs. $V-I$ CMDs for 32 BLAPs detected in the OGLE-IV outer
Galactic bulge fields are shown in Figs. 3–9. In the case of object OGLE-
BLAP-082, a CMD is not provided as it does not have the $(BP-RP)$ color in the
Gaia data, and DECaPS DR2 does not cover the vicinity of this star. Instead,
in Fig. 10, we present a color image of a nearby region from Pan-STARRS DR1
(Chambers et al. 2016).

Fig. 2. Period vs. I-band amplitude diagram for all 94 BLAPs detected in the
OGLE-IV fields. Positions of the 33 BLAPs from the outer Galactic bulge
presented in this work are marked with filled cyan circles. Twenty-five stars
from the Galactic disk fields (including the prototype object OGLE-BLAP-001)
are shown with blue empty squares. Filled blue triangles mark the positions of
36 BLAPs found in the inner Galactic bulge fields. $\delta$ Sct-type stars
from the OGLE fields for the Milky Way (bulge and disk) are represented with
gray points.

Some of the detected objects exhibit blur in their light curves, which is
likely caused by period changes. This can be seen in the case of OGLE-BLAP-085
and OGLE-BLAP-091. We note that the majority of observations for these stars
comes from seasons 2013, 2014, and 2023. The discovered BLAPs exhibit
characteristic light curve shapes, blue colors, and large amplitudes. The
periods of the identified objects range from 7.5 to 66.5 min and the I-band
amplitudes range from 0.11 to 0.43 mag. Object OGLE-BLAP-089, with a period of
7.51 min, supersedes OGLE-BLAP-044 ($P$ = 8.42 min) as the shortest BLAP
discovered in the OGLE data. Among the presented objects, OGLE-BLAP-069, has
the longest period of 66.49 min. Several distinct features can be recognized
in the phased light curves of the identified objects, with the majority of
objects showing classical sawtooth-like shape. The next group of light curves
is characterized by a sinusoidal-like bump after the maxiumum lasting for
about half of the cycle. A similar effect is observed in extreme helium stars
BX Cir ($P$ = 153.6 min) and V652 Her ($P$ = 155.4 min) (Kilkenny et al.
1999), although these stars have significantly smaller amplitudes. Examples of
objects exhibiting this effect are OGLE-BLAP-071 and OGLE-BLAP-083. Similar
stars were also found in the Galactic disk (e.g., OGLE-BLAP-043; Borowicz et
al. 2023) and in the inner bulge fields (e.g., OGLE-BLAP-019; Pietrukowicz et
al. 2024). Light curves of some BLAPs show an inverted shape, where the rising
branch lasts longer than the fading one (OGLE-BLAP-069 and OGLE-BLAP-085 in
our sample), similar to the effect observed in OGLE-BLAP-022 from the inner
bulge area. Another noticeable feature in some light curves is a
characteristic dip near the maximum brightness. This effect is visible in
object OGLE-BLAP-009 in an inner Galactic bulge field (Pietrukowicz et al.
2017) and OGLE-BLAP-040 in a Galactic disk field (Borowicz et al. 2023). The
additional features are more common for longer period objects, particularly
for periods longer than 50 min.

Based on the Gaia EDR3 parallaxes (Gaia Collaboration et al. 2021), we can
conclude that all the BLAPs reported here are located at distances of several
kpc from the Sun, similarly to previously found objects. We note that there
are no additional periodic signals down to amplitudes of 0.02 mag in the
detected stars, although this can result from a relatively low number of
brightness measurements.

###  4.2 Other pulsation-like variables

During the search, we found eighteen short-period variables with light curve
shapes resembling pulsating stars, although the asymmetry is less pronounced
than in the case of BLAPs. Figs. 11–14 shows phase-folded light curves and
CMDs with positions of the variables. Since there are no available $(BP-RP)$
measurements in the Gaia database, we used g and i-band data obtained by the
DECam Plane Survey to construct the CMDs for BLG882.04.34201 and
BLG918.29.51389. We note that we did not find any other periodicities in the
variables. For object BLG688.16.858, a distinct change in the period over the
course of the observations can be noted, as its light curve is blurred. All
the reported variables are located blueward of the main sequence. In Table 2,
we list the observational properties of the stars ordered with the increasing
right ascension. The parallax values around and below 1 mas and I-band
brightnesses in range 15.5-19 mag indicate that the objects are not pulsating
white dwarfs. The variables are very likely of $\delta$ Sct/SX Phe type.
However, some of them can be pulsating subdwarfs of spectral types OB (sdOB).
We cannot rule out a possibility that there are BLAPs in this sample. Only
object BLG481.23.763 was assigned a variability class in the Gaia DR3 (as
DSCT$|$GDOR$|$SXPHE).

Interestingly, variable BLG954.25.16264 is located at an angular distance of
only 4.′69 from the center of globular cluster NGC 6541. This is about 4.42
times its half-light radius ($r_{\rm h}=1$.′06, according to the 2010 edition
of a catalog of parameters for Milky Way’s globular clusters prepared by
Harris 1996). On the CMD, the star falls in the area where the cluster blue
stragglers reside. Very likely, object BLG954.25.16264 is a SX Phe-type
variable that belongs to NGC 6541.

###  4.3 Eclipsing Binary Systems

Our analysis of the OGLE-IV outer Galactic bulge dataset reveals five short-
period eclipsing binary systems. These findings are illustrated in Fig. 15,
where we present a CMD alongside the folded light curve for each system
according to its orbital period. Furthermore, Table 3 provides detailed
photometric properties for the objects. Only one of them, BLG764.18.16589, was
classified as an eclipsing variable (ECL type) in the Gaia DR3, but with an
incorrect period. Systems BLG723.31.34651 and BLG942.13.1292 may belong to the
class of recently discovered Roche lobe-filling hot subdwarf binaries (Kupfer
et al. 2020b). Their properties resemble those of objects ZTF J2130+4420
(Kupfer et al. 2020a) and ZTF J2055+4651 (Kupfer et al. 2020b), which are
confirmed sdOB+WD binaries. Object BLG764.18.16598 with the period of 82.43
min is likely a HW Vir-type system with a strong reflection effect. This star
was previously found as ZTF J1810-2138 and classified as a detached binary
(Ren et al. 2023). System BLG608.02.2325 with the orbital period of 61.23 min
could belong to one of the aforementioned types.

An intriguing object is BLG832.03.1534 with the orbital period of 121.86 min.
The shape of the phased light curve with the minima of similar depth points to
a contact binary system. Although currently, shorter-period hot contact
binaries are known, the location of this system on the CMD suggests that it
does not contain a hot subdwarf. Contact binaries with orbital periods below
0.2 d and formed of red dwarfs are very rare. Example systems at the short-
period end are WFCAM 19b-3-06008 with $P_{\rm orb}\approx 161$ min (Nefs et
al. 2012) and OGLE-BLG-ECL-000066 with $P_{\rm orb}\approx 141$ min (Soszyński
et al. 2015), making BLG832.03.1534 a good candidate for the shortest known
cool contact binary. With a parallax of 5.6 mas, the object is located
relatively close to the Sun compared to the remaining binary systems reported
here, thus it is likely composed of M dwarfs.

###  4.4 Mysterious short-period variables

Our search led to the detection of the following three unusual variables shown
in Fig. 16. Object BLG696.13.32988 has a huge amplitude of about 1.6 mag at
the extremely short period of 11.20 min. Its light curve seems to be
symmetric. We inform that this star is a faint but isolated object in the OGLE
images. The other two variables, BLG704.31.1645 and BLG391.22.1563, have very
bizarre light curve shapes at a period of about 90 min. Their properties are
very similar to one of the previously discovered objects in the OGLE-IV
Galactic disk fields, GD2117.15.7515 with the period of $P=90.74$ min
(Borowicz et al. 2023). Object BLG391.22.1563 with the huge I-band amplitude
of about 2.5 mag was classified as a cataclysmic variable (CV) in Gaia DR3,
which is a possible explanation for its nature. However, none of the objects
reported in this work has an X-ray counterpart.

##  5 Summary

The presented study focused on the search for short-period ($<$ 1 h) variable
stars in the OGLE-IV outer Galactic bulge fields. We put a specific emphasis
on the detection of BLAPs and compact binary systems. The monitoring of about
700 million stellar sources allowed us to detect around three hundred periodic
variable sources, among which 33 are genuine BLAPs and five are eclipsing
binary systems. Eighteen other of the detected large-amplitude ($>$ 0.1 mag)
variables are likely pulsating stars. We also found three variables of an
uncertain type that have a huge amplitude ($>$ 0.9 mag). The remaining
detected objects are low-amplitude variables of unassigned type.

Our investigation have elevated the number of BLAPs from OGLE to 94, bringing
the total number of these stars to over one hundred. OGLE-BLAP-092 with a
pulsation period of 7.51 min has replaced OGLE-BLAP-044 with $P$ = 8.42 min as
the shortest-period BLAP discovered by OGLE. Some of the newly identified
objects exhibit unusual features such as bumps or dips in their light curves.
The light curves of objects OGLE-BLAP-085, OGLE-BLAP-091, and OGLE-BLAP-093
show noticeable alterations in their shape, which could be caused by a period
changes. The substantial increase in the population of BLAPs allows us to put
a better constraint on their parameters and provides targets for further
investigations.

In addition to the BLAPs, we identified five short-period eclipsing binary
systems with orbital periods in the range 61–121 min. These objects likely
contain typical blue reflection-effect binaries of HW Vir type and compact sdB
systems with ellipsoidal modulation, whereas objects BLG832.03.1534 is
unusually red and has a very short period for a possible EW-type binary
system. Two of the objects in our sample could be cataclysmic variables.
Eighteen other variables with pulsation-like light curves could be BLAPs,
short-period $\delta$ Sct stars or oscillating subdwarfs. Photometry alone is
insufficient to determine the class of these variables. Follow-up
spectroscopic observations would allow their final classification.

Fig. 3. Phase-folded I-band light curves (left panels) and CMDs (right panels)
for BLAPs detected in the OGLE-IV fields for the outer Galactic bulge. The
diagrams were constructed using data from Gaia DR3 within $0.1\times 0.1$
square-degree regions centered on the variable objects. Positions of the BLAPs
in the CMDs are marked with blue dots. Note the presence of an additional bump
in the light curve of object OGLE-BLAP-063.

Fig. 4. Same as Fig. 3 for another five BLAPs. Object OGLE-BLAP-070 exhibits a
characteristic dip around the maximum light, which is present in several other
BLAPs including the best-studied case so far, OGLE-BLAP-009 (Bradshaw et al.
2024).

Fig. 5. Same as Fig. 3 for another five BLAPs.

Fig. 6. Same as Fig. 3 for another five BLAPs.

Fig. 7. Same as Fig. 3 for another five BLAPs. A prominent additional bump is
present in the light curve of OGLE-BLAP-083.

Fig. 8. Same as Fig. 3 for another five BLAPs. In the case of OGLE-BLAP-091,
the blurred light curve is a result of period variations over a decade-long
observations. Object OGLE-BLAP-092 has the shortest pulsation period in the
whole BLAP collection from OGLE.

Fig. 9. Phase-folded I-band light curves along with Gaia DR3 CMDs for two
remaining BLAPs. An unusual scatter can be observed in the light curve of
OGLE-BLAP-093.

Fig. 10. Phase-folded I-band light curve of OGLE-BLAP-082 (left panel) and a
22”$\times$14” color chart cropped from a Pan-STARRS image with this variable
star (right panel). Position of the star is marked with a green cross.

Fig. 11. Phase-folded I-band light curves (first column) and CMDs (second
column) for four pulsation-like variable stars detected in the OGLE-IV fields
for the outer Galactic bulge.

Fig. 12. Same as Fig. 11 for another five objects. In the case of object
BLG688.16.858, a period change results in the blurred light curve.

Fig. 13. Same as Fig. 11 for another five objects. The CMDs for
BLG882.04.34201 is based on g and i-band brightness measurements obtained from
the DECam Plane Survey.

Fig. 14. Same as Fig. 11 for another four objects. The CMDs for
BLG918.29.51389 is based on g and i-band brightness measurements obtained from
the DECam Plane Survey.

Fig. 15. Phase-folded I-band light curves (left panels) and CMDs (right
panels) for five eclipsing binary systems found in the OGLE-IV outer Galactic
bulge fields.

Fig. 16. Phase-folded I-band light curves and CMDs for three large-amplitude
variables of unknown type. Objects BLG391.22.1563 and BLG696.13.32988 have
minimum brightness near the detection limit in the shallow outer bulge survey.

Table 1: Photometric properties of detected BLAPs

Name | RA (J2000) | Dec (J2000) | $l$ | $b$ | $\varpi$ | $\sigma_{\varpi}$ | $P_{\rm puls}$ | $\sigma_{P}$ | $<I>$ | $A_{I}$ | $<G>$ | $BP-RP$ | Other Name
---|---|---|---|---|---|---|---|---|---|---|---|---|---
|  [∘] |  [∘] |  [∘] |  [∘] | [mas] | [mas] | [min] | [$10^{-7}$ min] | [mag] | [mag] | [mag] | [mag] | [Ref.]
OGLE-BLAP-062 | 254.83294 | $-37.11583$ | 347.76988 | 3.34321 | — | — | 32.9956511 | 106 | 17.986 | 0.241 | 18.835 | 1.365 | –
OGLE-BLAP-063 | 256.51623 | $-31.67478$ | 352.95038 | 5.54725 | 0.30 | 0.10 | 58.6452979 | 33 | 16.744 | 0.155 | 17.275 | 0.723 | –
OGLE-BLAP-064 | 258.32242 | $-33.58885$ | 352.30587 | 3.18846 | — | — | 40.4852661 | 39 | 18.124 | 0.258 | 18.745 | 1.068 | –
OGLE-BLAP-065 | 260.88549 | $-43.96965$ | 344.95315 | $-4.41049$ | 0.07 | 0.18 | 15.1879302 | 7 | 17.817 | 0.292 | 18.365 | 0.771 | –
OGLE-BLAP-066 | 261.29448 | $-39.93427$ | 348.47142 | $-2.39965$ | 0.28 | 0.56 | 27.9819187 | 58 | 18.509 | 0.233 | 19.461 | 1.609 | –
OGLE-BLAP-067 | 261.91435 | $-40.58023$ | 348.19822 | $-3.15237$ | 0.02 | 0.25 | 20.3038847 | 103 | 18.282 | 0.376 | 18.704 | 0.577 | –
OGLE-BLAP-068 | 262.38303 | $-24.60972$ | 1.74050 | 5.34600 | 1.18 | 0.30 | 38.3321045 | 72 | 17.956 | 0.192 | 18.943 | 1.555 | –
OGLE-BLAP-069 | 262.57431 | $-20.65069$ | 5.18119 | 7.34917 | 0.26 | 0.09 | 66.4939433 | 646 | 16.825 | 0.123 | 17.461 | 0.912 | –
OGLE-BLAP-070 | 262.96609 | $-23.06989$ | 3.33021 | 5.73570 | 0.49 | 0.24 | 34.8664728 | 33 | 17.413 | 0.213 | 18.170 | 1.544 | –
OGLE-BLAP-071 | 263.23020 | $-40.16291$ | 349.09850 | $-3.76193$ | 0.29 | 0.05 | 58.4985243 | 30 | 15.113 | 0.154 | 15.577 | 0.648 | –
OGLE-BLAP-072 | 264.47945 | $-18.04700$ | 8.36775 | 7.21315 | 0.10 | 0.09 | 31.8847179 | 36 | 16.976 | 0.326 | 17.169 | 0.391 | –
OGLE-BLAP-073 | 265.45330 | $-20.85852$ | 6.44301 | 4.95813 | — | — | 36.0712030 | 39 | 17.397 | 0.264 | 18.126 | 1.073 | –
OGLE-BLAP-074 | 265.56943 | $-16.17125$ | 10.52892 | 7.29922 | 0.14 | 0.15 | 15.3685860 | 11 | 18.057 | 0.286 | 18.177 | 0.184 | –
OGLE-BLAP-075 | 265.65443 | $-38.87139$ | 351.19269 | $-4.65797$ | 0.28 | 0.18 | 22.0857709 | 15 | 18.133 | 0.317 | 18.582 | 0.697 | –
OGLE-BLAP-076 | 267.18553 | $-19.57779$ | 8.38603 | 4.22924 | 0.16 | 0.19 | 34.3470343 | 71 | 17.902 | 0.268 | 18.506 | 1.044 | –
OGLE-BLAP-077 | 268.08081 | $-16.78669$ | 11.23146 | 4.91379 | 0.20 | 0.17 | 10.1121423 | 4 | 17.728 | 0.236 | 18.379 | 0.961 | –
OGLE-BLAP-078 | 268.13777 | $-16.73893$ | 11.30044 | 4.89074 | 0.48 | 0.32 | 22.1350836 | 21 | 18.585 | 0.428 | 19.166 | 1.034 | –
OGLE-BLAP-079 | 269.19956 | $-19.62019$ | 9.31129 | 2.57134 | 0.55 | 0.20 | 41.3926356 | 33 | 17.526 | 0.219 | 18.574 | 1.642 | –
OGLE-BLAP-080 | 269.26293 | $-16.26118$ | 12.25711 | 4.19601 | 0.90 | 0.35 | 21.6268702 | 26 | 18.530 | 0.295 | 19.278 | 1.112 | –
OGLE-BLAP-081 | 269.62850 | $-14.52371$ | 13.94580 | 4.75164 | 0.47 | 0.18 | 38.5197428 | 345 | 17.239 | 0.243 | 18.413 | 1.983 | –
OGLE-BLAP-082 | 270.51874 | $-17.78211$ | 11.52927 | 2.39961 | — | — | 53.5543446 | 124 | 17.249 | 0.201 | 17.930 | — | –
OGLE-BLAP-083 | 275.79400 | $-24.11316$ | 8.32787 | $-4.95472$ | 0.20 | 0.10 | 47.7527083 | 52 | 16.942 | 0.180 | 17.543 | 0.917 | –
OGLE-BLAP-084 | 276.07003 | $-20.11729$ | 11.99588 | $-3.32710$ | — | — | 34.2030899 | 32 | 17.654 | 0.264 | 18.393 | 1.099 | –
OGLE-BLAP-085 | 276.25389 | $-24.66896$ | 8.02645 | $-5.58261$ | 0.07 | 0.08 | 46.0804344 | 87 | 16.780 | 0.182 | 17.104 | 0.594 | –
OGLE-BLAP-086 | 276.61851 | $-28.20053$ | 5.01152 | $-7.48189$ | 0.01 | 0.08 | 32.9633868 | 47 | 16.735 | 0.226 | 16.813 | 0.625 | [1]
OGLE-BLAP-087 | 277.97158 | $-18.85708$ | 13.94509 | $-4.33657$ | 0.04 | 0.15 | 20.4200231 | 13 | 17.370 | 0.330 | 17.899 | 0.721 | –
OGLE-BLAP-088 | 278.01615 | $-21.00101$ | 12.04936 | $-5.35152$ | 0.33 | 0.21 | 20.5911629 | 8 | 17.873 | 0.361 | 18.208 | 0.423 | –
OGLE-BLAP-089 | 278.01967 | $-19.70939$ | 13.20528 | $-4.76625$ | 0.16 | 0.15 | 25.1626966 | 19 | 17.608 | 0.290 | 18.013 | 0.459 | –
OGLE-BLAP-090 | 278.42267 | $-13.30536$ | 19.08535 | $-2.17429$ | — | — | 55.4221598 | 58 | 16.226 | 0.114 | 16.901 | 1.084 | –
OGLE-BLAP-091 | 279.09657 | $-16.54557$ | 16.49809 | $-4.23598$ | 0.54 | 0.03 | 22.5836372 | 52 | 14.767 | 0.124 | 15.105 | 0.400 | –
OGLE-BLAP-092 | 279.74064 | $-22.74435$ | 11.20470 | $-7.56414$ | — | — | 7.5140274 | 3 | 17.988 | 0.158 | 17.958 | 0.238 | –
OGLE-BLAP-093 | 281.27844 | $-30.13465$ | 5.03473 | $-11.98657$ | 0.12 | 0.06 | 20.1679637 | 197 | 16.601 | 0.125 | 16.476 | $-0.248$ | [2]
OGLE-BLAP-094 | 281.80659 | $-17.13507$ | 17.14000 | $-6.81803$ | 0.21 | 0.06 | 25.3925666 | 26 | 16.496 | 0.357 | 16.777 | 0.339 | –

References: [1] OW J1826-2812 (Ramsay et al. 2022), [2] SMSS
J184506.82-300804.7 (Chang et al. 2024)

Table 2: Photometric properties of eighteen pulsating-like stars

Name | RA (J2000) | Dec (J2000) | $l$ | $b$ | $\varpi$ | $\sigma_{\varpi}$ | $P_{\rm puls}$ | $\sigma_{P}$ | $<I>$ | $A_{I}$ | $<G>$ | $BP-RP$
---|---|---|---|---|---|---|---|---|---|---|---|---
|  [∘] |  [∘] |  [∘] |  [∘] | [mas] | [mas] | [min] | [$10^{-7}$ min] | [mag] | [mag] | [mag] | [mag]
BLG852.09.20745 | 251.16834 | $-29.75587$ | 351.60113 | $10.34601$ | — | — | 53.0253606 | 444 | 18.323 | 0.258 | 18.774 | 0.668
BLG481.23.763 | 254.35439 | $-21.53726$ | 0.01575 | $13.16083$ | 0.01 | 0.16 | 47.9596203 | 437 | 17.526 | 0.173 | 18.069 | 0.775
BLG886.28.45247 | 254.79290 | $-32.31533$ | 351.54807 | $6.32243$ | — | — | 51.2152787 | 66 | 17.671 | 0.214 | 18.227 | 0.829
BLG882.04.34201 | 254.82191 | $-28.35160$ | 354.73151 | $8.72091$ | 0.15 | 0.17 | 52.3152169 | 670 | 17.582 | 0.173 | 18.016 | —
BLG887.11.1867 | 254.84920 | $-34.23643$ | 350.05285 | $5.10612$ | — | — | 42.7895905 | 57 | 17.218 | 0.104 | 17.971 | 1.080
BLG913.15.4196 | 258.42735 | $-25.00119$ | 359.38741 | $8.10018$ | 0.18 | 0.27 | 53.7033771 | 535 | 18.189 | 0.298 | 19.049 | 1.221
BLG918.29.51389 | 259.04021 | $-32.86844$ | 353.24256 | $3.11955$ | 0.43 | 0.66 | 17.2779357 | 34 | 19.022 | 0.258 | 20.066 | —
BLG807.31.13428 | 262.17954 | $-14.87571$ | 9.91594 | $10.76036$ | 0.23 | 0.13 | 52.4995780 | 493 | 17.257 | 0.157 | 17.882 | 0.870
BLG967.30.3692 | 262.31942 | $-23.61095$ | 2.54932 | $5.94017$ | — | — | 32.5423393 | 78 | 18.165 | 0.174 | 19.631 | 1.928
BLG537.24.71551 | 268.79063 | $-37.55382$ | 353.59142 | $-6.10845$ | — | — | 51.5481337 | 107 | 18.122 | 0.206 | 18.656 | 0.660
BLG942.19.26628 | 269.79992 | $-41.34727$ | 350.63784 | $-8.64194$ | 1.09 | 0.14 | 45.0608030 | 902 | 17.665 | 0.212 | 18.004 | 0.460
BLG541.09.33260 | 271.46953 | $-38.60803$ | 353.68966 | $-8.46380$ | 0.32 | 0.05 | 47.5686369 | 80 | 15.487 | 0.130 | 15.820 | 0.479
BLG954.25.16264 | 271.92296 | $-43.76137$ | 349.21479 | $-11.15339$ | 0.28 | 0.13 | 52.6039070 | 680 | 17.607 | 0.214 | 17.936 | 0.427
BLG658.19.4003 | 275.32101 | $-31.62993$ | 1.41166 | $-8.03147$ | 0.22 | 0.17 | 49.8625157 | 259 | 17.793 | 0.138 | 18.172 | 0.596
BLG688.16.858 | 275.72691 | $-32.46720$ | 0.81361 | $-8.71653$ | 0.01 | 0.05 | 43.6297893 | 238 | 16.267 | 0.154 | 16.234 | 0.022
BLG382.24.180 | 276.30373 | $-35.99486$ | 357.81897 | $-10.70649$ | 0.29 | 0.05 | 52.7114147 | 3579 | 15.626 | 0.137 | 15.867 | 0.377
BLG555.08.43224 | 276.94071 | $-28.75299$ | 4.64216 | $-7.98335$ | 0.22 | 0.20 | 47.3230106 | 229 | 18.126 | 0.182 | 18.665 | 0.689
BLG399.06.732 | 281.14731 | $-26.74430$ | 8.12033 | $-10.45424$ | 0.16 | 0.06 | 40.1926896 | 288 | 16.519 | 0.103 | 16.494 | $-0.025$

Table 3: Photometric properties of binary systems and other high-amplitude
variables

Name | RA (J2000) | Dec (J2000) | $l$ | $b$ | $\varpi$ | $\sigma_{\varpi}$ | P | $\sigma_{P}$ | $<I>$ | $A_{I}$ | $<G>$ | $BP-RP$
---|---|---|---|---|---|---|---|---|---|---|---|---
|  [∘] |  [∘] |  [∘] |  [∘] | [mas] | [mas] | [min] | [$10^{-7}$ min] | [mag] | [mag] | [mag] | [mag]
BLG608.02.2325 | 266.58028 | $-40.91053$ | 349.80731 | $-6.31960$ | 0.48 | 0.09 | 61.2255171 | 38 | 17.309 | 0.354 | 17.425 | 0.117
BLG832.03.1534 | 267.78860 | $-15.99452$ | 11.77647 | $5.55531$ | 5.61 | 0.55 | 121.862758 | 423 | 16.414 | 0.847 | 18.157 | 2.242
BLG942.13.1292 | 269.33622 | $-41.70733$ | 350.14952 | $-8.51156$ | 0.30 | 0.06 | 79.9752372 | 522 | 16.564 | 0.243 | 16.497 | $-0.023$
BLG764.18.16598 | 272.55705 | $-21.64039$ | 9.09754 | $-1.15788$ | 0.62 | 0.11 | 82.4258726 | 107 | 16.979 | 0.457 | 17.543 | 0.723
BLG723.31.34651 | 280.85077 | $-17.02810$ | 16.82668 | $-5.95244$ | 0.44 | 0.06 | 63.8959988 | 21 | 16.269 | 0.467 | 16.440 | 0.184
BLG696.13.32988 | 277.79776 | $-34.18815$ | 0.01607 | $-11.02687$ | — | — | 11.1984346 | 56 | 19.680 | 1.629 | 20.169 | 0.792
BLG704.31.1645 | 279.19703 | $-34.29750$ | 0.41518 | $-12.12097$ | 0.50 | 0.12 | 89.7870609 | 1014 | 17.337 | 0.908 | 17.516 | 0.118
BLG391.22.1563 | 279.87395 | $-37.19227$ | 357.93805 | $-13.81055$ | 1.04 | 0.43 | 92.6047602 | 3496 | 18.850 | 2.536 | 19.585 | 0.901

Acknowledgments. We thank all the OGLE observers for their contribution to the
collection of the photometric data over the decades. This work has been funded
by the National Science Centre, Poland, grant no. 2022/45/B/ST9/00243 to I.S.
For the purpose of Open Access, the author has applied a CC-BY public
copyright licence to any Author Accepted Manuscript (AAM) version arising from
this submission. We used data from the European Space Agency (ESA) mission
Gaia, processed by the Gaia Data Processing and Analysis Consortium (DPAC).
Funding for the DPAC has been provided by national institutions, in particular
the institutions participating in the Gaia Multilateral Agreement.

## REFERENCES

  * Barlow, B. N., Corcoran, K. A., Parker, I. M., et al.  2022, Astrophys. J., 928, 20.

  * Bellm, E. C., Kulkarni, S. R., Graham, M. J., et al.  2019, P.A.S.P., 131, 018002.

  * Borowicz, J., Pietrukowicz, P., Mróz, P., et al.  2023, Acta Astron., 73, 1.

  * Bradshaw, C. W., Dorsch, M., Kupfer, T., et al.  2024, MNRAS, 527, 10239.

  * Burdge, K. B., Coughlin, M. W., Fuller, J., et al.  2019, Nature, 571, 528.

  * Burdge, K. B., Prince, T. A., Fuller, J., et al.  2020, Astrophys. J., 905, L7.

  * Byrne, C. M., Jeffery, C. S. 2018, MNRAS, 481, 3810.

  * Byrne, C. M., Stanway, E. R., and Eldridge, J. J. 2021, MNRAS, 507, 621.

  * Chambers, K. C., Magnier, E. A., Metcalfe, N., et al.  2016, arXiv e-prints, arXiv:1612.05560.

  * Chang, S.-W., Wolf, C., Onken, C. A., and Bessell, M. S. 2024, MNRAS, 529, 1414.

  * Gaia Collaboration, Brown, A. G. A., et al.  2021, Astron. Astrophys., 649, A1.

  * Harris, W. E. 1996, Astron. J., 112, 1487.

  * Iwanek, P., Soszyński, I., Kozłowski. S., et al.  2022, Astrophys. J. Suppl. Ser., 260, 46.

  * Kilkenny, D., Koen, C., Jeffery, C. S., Hill, N. C., & O’Donoghue, D. 1999, MNRAS, 310, 1119.

  * Koen, C. 2024, PASA, 41, e010.

  * Kupfer, T., Bauer, E. B., Burdge, K. B., et al.  2019, Astrophys. J., 878, L35.

  * Kupfer, T., Bauer, E. B., Marsh, T. R., et al.  2020a, Astrophys. J., 891, 45.

  * Kupfer, T., Bauer, E. B., Burdge, K. B., et al.  2020b, ApJL, 898, L25.

  * Lin, J., Wu, C., Wang, X., et al.  2023a, Nature Astronomy, 7, 223.

  * Lin, J., Wu, C., Wang, X.,et al.  2023b, MNRAS, 523, 2172.

  * Meng, X.-C., Han, Z.-W., Podsiadlowski, P., and Li, J. 2020, Astrophys. J., 903, 100.

  * McWhirter, P. R., and Lam, M. C. 2022, MNRAS, 511, 4971.

  * Nefs, S. V., Birkby, J. L., Snellen, I. A. G., et al.  2012, MNRAS, 425, 950.

  * Onken, C. A., Wolf, C.; Bessell, M. S., et al.  2019, Publications of the Astronomical Society of Australia, 36, 33.

  * Pietrukowicz, P., Dziembowski, W. A., Latour, M., et al.  2017, Nature Astronomy, 1, 0166.

  * Pietrukowicz, P., Ratajczak, M., Soszyński, I., et al.  2022, Acta Astron., 72, 161.

  * Pietrukowicz, P., Latour, M., Soszyński, I., et al.  2024, arXiv e-prints, arXiv:2404.16089.

  * Pigulski, A., Kotysz, K., and Kołaczek-Szymański, P. A. 2022, Astron. Astrophys., 663, A62.

  * Ramsay, G., Woudt, P. A., Kupfer, T., et al.  2022, MNRAS, 513, 2215.

  * Ren, L., Li, C., Ma, B., et al.  2023, Astrophys. J. Suppl. Ser., 264, 39.

  * Romero, A. D., Córsico, A. H., Althaus, L. G., Pelisoli, I., and Kepler, S. O. 2018, MNRAS, 477, L30.

  * Saydjari, A. K., Schlafly, E. F., Lang, D., et al.  2023, Astrophys. J. Suppl. Ser., 264, 28.

  * Soszyński, I., Stépień, K., Pilecki, B., et al.  2015, Acta Astron., 65, 39.

  * Soszyński, I., Udalski, A., Wrona, M., et al.  2019, Acta Astron., 69, 321.

  * Soszyński, I., Pietrukowicz, P., Skowron, J., et al.  2021, Acta Astron., 71, 189.

  * Schwarzenberg-Czerny, A. 1996, Astron. J., 460, L107.

  * Udalski, A., Szymański, M. K., and Szymański, G. 2015, Acta Astron., 65, 1.

  * Udalski, A., Soszyński, I., Pietrukowicz, P., et al.  2018, Acta Astron., 68, 315.

  * Wu, T., and Li, Y. 2018, MNRAS, 478, 3871.

  * Xiong, H., Casagrande, L., Chen, X., et al.  2022, Astron. Astrophys., 668, A112.

  * Zhang, X., Jeffery, C. S., Su, J., and Bi, S. 2023, Astrophys. J., 959, 24.