Short-period Variables Research Articles

Short-period Variables in TESS Full-frame Image Light Curves Identified via Convolutional Neural Networks

The Transiting Exoplanet Survey Satellite (TESS) mission measured light from stars in ∼85% of the sky throughout its 2 yr primary mission, resulting in millions of TESS 30-minute-cadence light curves to analyze in the search for transiting exoplanets. To search this vast data set, we aim to provide an approach that is computationally efficient, produces accurate predictions, and minimizes the required human search effort. We present a convolutional neural network that we train to identify short-period variables. To make a prediction for a given light curve, our network requires no prior target parameters identified using other methods. Our network performs inference on a TESS 30-minute-cadence light curve in ∼5 ms on a single GPU, enabling large-scale archival searches. We present a collection of 14,156 short-period variables identified by our network. The majority of our identified variables fall into two prominent populations, one of close-orbit main-sequence binaries and another of δ Scuti stars. Our neural network model and related code are additionally provided as open-source code for public use and extension.

Searching for Short-period Variables in M31: Method and Catalogs

Utilizing high-cadence and continuous g- and r-band data over three nights acquired from the 3.6 m Canada–France–Hawaii Telescope, aiming to find short-duration microlensing events, we conduct a systematic search for variables, transients, and asteroids across a ∼1° field of view of the Andromeda galaxy (M31). We present a catalog of 5859 variable stars, yielding the most extensive compilation of short-period variable sources of M31. We also detected 19 flares, predominantly associated with foreground M dwarfs in the Milky Way. In addition, we discovered 17 previously unknown asteroid candidates, and we subsequently reported them to the Minor Planet Center. Lastly, we report a microlensing event candidate C-ML-1 and present a preliminary analysis.

Minute-Cadence Observations of the LAMOST Fields with the TMTS: V. Machine Learning Classification of TMTS Catalogues of Periodic Variable Stars

Abstract Periodic variables are always of great scientific interest in astrophysics. Thanks to the rapid advancement of modern large-scale time-domain surveys, the number of reported variable stars has experienced substantial growth for several decades, which significantly deepened our comprehension of stellar structure and binary evolution. The Tsinghua University-Ma Huateng Telescopes for Survey (TMTS) has started to monitor the LAMOST sky areas since 2020, with a cadence of 1 minute. During the period from 2020 to 2022, this survey has resulted in densely sampled light curves for ~30,000 variables of the maximum powers in the Lomb-Scargle periodogram above the 5σ threshold. In this paper, we classified 11,638 variable stars into 6 main types using XGBoost and Random Forest classifiers with accuracies of 98.83% and 98.73%, respectively. Among them, 5301 (45.55%) variables are newly discovered, primarily consisting of δ Scuti stars, demonstrating the capability of TMTS in searching for short-period variables. We cross-matched the catalogue with Gaia’s second Data Release (DR2) and LAMOST’s seventh Data Release (DR7) to obtain important physical parameters of the variables. We identified 5504 δ Scuti stars (including 4876 typical δ Scuti stars and 628 high-amplitude δ Scuti stars), 5899 eclipsing binaries (including EA-, EB- and EW-type) and 226 candidates of RS Canum Venaticorum. Leveraging the metal abundance data provided by LAMOST and the Galactic latitude, we discovered 8 candidates of SX Phe stars within the class of ”δ Scuti stars”. Moreover, with the help of Gaia color-magnitude diagram, we identified 9 ZZ ceti stars.

GaiaData Release 3

As part ofGaiaData Release 3, a large number of ellipsoidal variables were identified with supervised classification. The periodic variability of these ellipsoidals is presumably induced by tidal interaction with a companion in a close binary system. We present 6306 short-period probable ellipsoidal variables with relatively large-amplitudeGaiaG-band photometric modulations that indicate a possible massive, undetected secondary. In case of a main-sequence primary, the more massive secondary is probably a compact object – either a black hole or a neutron star, and sometimes a white dwarf. The identification is based on the recently suggested robust modified minimum mass ratio (mMMR) that was derived from the observed ellipsoidal amplitude alone, without the use of the primary mass or radius. We also list a subset of 262 systems with an mMMR higher than unity, for which the probability of a compact secondary is higher. Follow-up observations are needed to verify the true nature of these variables.

Minute-cadence observations of the LAMOST fields with the TMTS: II. Catalogues of short-period variable stars from the first 2-yr surveys

ABSTRACT Over the past few years, wide-field time-domain surveys such as Zwicky Transient Facility and Optical Gravitational Lensing Experiment have led to discoveries of various types of interesting short-period stellar variables, such as ultracompact eclipsing binary white dwarfs (WDs), rapidly rotating magnetized WDs, transitional cataclysmic variables between hydrogen-rich and helium accretion, and blue large-amplitude pulsators (BLAPs), which greatly enrich our understandings of stellar physics under some extreme conditions. In this paper, we report the first-2-yr discoveries of short-period variables (i.e. P < 2 h) by the Tsinghua University–Ma Huateng Telescopes for Survey (TMTS). TMTS is a multitube telescope system with a field of view up to 18 deg2, which started to monitor the Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST) sky areas since 2020 and generated uninterrupted minute-cadence light curves for about 10 million sources within 2 yr. Adopting the Lomb–Scargle periodogram with period-dependent thresholds for the maximum powers, we identify over 1100 sources that exhibit a variation period shorter than 2 h. Compiling the light curves with the Gaia magnitudes and colours, LAMOST spectral parameters, International Variable Star Index classifications, and archived observations from other prevailing time-domain survey missions, we identified 1076 as δ Scuti stars, which allows us to study their populations and physical properties in the short-period regime. The other 31 sources include BLAPs, subdwarf B variables, pulsating WDs, ultracompact/short-period eclipsing/ellipsoidal binaries, cataclysmic variables below the period gap, etc., which are highly interesting and worthy of follow-up investigations.

Minute-cadence observations of the LAMOST fields with the TMTS – III. Statistical study of the flare stars from the first two years

ABSTRACTTsinghua University-Ma Huateng Telescopes for Survey (TMTS) aims to detect fast-evolving transients in the Universe, which has led to the discovery of thousands of short-period variables and eclipsing binaries since 2020. In this paper, we present the observed properties of 125 flare stars identified by TMTS within the first two years, with an attempt to constrain their eruption physics. As expected, most of these flares were recorded in late-type red stars with GBP − GRP >2.0 mag; however, the flares associated with bluer stars tend to be on average more energetic and have broader profiles. The peak flux (Fpeak) of the flare is found to depend strongly on the equivalent duration (ED) of the energy release, i.e. Fpeak∝ED0.72 ± 0.04, which is consistent with results derived from the Kepler and Evryscope samples. This relation is likely to be related to the magnetic loop emission, while, for the more popular non-thermal electron heating model, a specific time evolution may be required to generate this relation. We notice that flares produced by hotter stars have a flatter Fpeak - ED relation compared to that from cooler stars. This is related to the statistical discrepancy in light-curve shape of flare events with different colours. In spectra from LAMOST, we find that flare stars have apparently stronger H α emission than inactive stars, especially at the low-temperature end, suggesting that chromospheric activity plays an important role in producing flares. On the other hand, the subclass with frequent flares is found to show H α emission of similar strength in its spectra to that recorded with only a single flare but similar effective temperature, implying that chromospheric activity may not be the only trigger for eruptions.

QT and Tpeak-Tend interval variability: Predictive electrical markers of hospital stay length and mortality in acute decompensated heart failure. Preliminary data.

As previously reported, an increased repolarization temporal imbalance induces a higher risk of total/cardiovascular mortality. The aim of this study was to assess if the electrocardiographic short period markers of repolarization temporal dispersion could be predictive of the hospital stay length and mortality in patients with acutely decompensated chronic heart failure (CHF). Mean, standard deviation (SD), and normalized variance (VN) of QT (QT) and Tpeak-Tend (Te) were obtained on 5-minECG recording in 139 patients hospitalized for acutely decompensated CHF, subgrouping the patients for hospital length of stay (LoS): less or equal 1 week (≤1 W) and those with more than 1week (>1 W). We observed an increase of short-period repolarization variables (TeSD and TeVN, p < .05), a decrease of blood pressure (p < .05), lower ejection fraction (p < .05), and higher plasma level of biomarkers (NT-proBNP, p < .001; Troponin, p < .05) in >1 W LoS subjects. 30-day deceased subjects reported significantly higher levels of QTSD (p < .05), Te mean (p < .001), TeSD (p < .05), QTVN (p < .05) in comparison to the survivors. Multivariable Cox regression analysis reported that TeVN was a risk factor for longer hospital stay (hazard ratio: 1.04, 95% confidence limit: 1.01-1.08, p < .05); whereas, a longer Te mean was associated with higher mortality risk (hazard ratio: 1.02, 95% confidence limit: 1.01-1.03, p < .05). A longer hospital stay is considered a clinical surrogate of CHF severity, we confirmed this finding. Therefore, these electrical and simple parameters could be used as noninvasive, transmissible, inexpensive markers of CHF severity and mortality.

System parameters of three short-period cataclysmic variable stars

ABSTRACT Using photometric ULTRACAM observations of three new short-period cataclysmic variables (CVs), we model the primary eclipse light curves to extract the orbital separation, masses, and radii of their component stars. We find donor masses of $0.060\pm 0.008\, {\rm M_\odot }$, $0.042\pm 0.001\, {\rm M_\odot }$, and $0.042\pm 0.004\, {\rm M_\odot }$, two being very low-mass sub-stellar donors, and one within 2σ of the hydrogen burning limit. All three of the new systems lie close to the empirical evolutionary sequence that has emerged from observations of the last decade. We briefly re-evaluate the long-standing discrepancy between observed donor mass and radius data, and theoretical CV evolutionary tracks. By looking at the difference in the observed period at each mass and the period predicted by the modelled evolutionary sequences, we qualitatively examine the form of excess angular momentum loss that is missing from the models below the period gap. We show indications that the excess angular momentum loss missing from CV models grows in importance relative to gravitational losses as the period decreases. Detailed CV evolutionary models are necessary to draw more quantitative conclusions in the future.

Minute-cadence Observations of the LAMOST Fields with the TMTS: I. Methodology of Detecting Short-period Variables and Results from the first-year Survey

Abstract Tsinghua University-Ma Huateng Telescopes for Survey (TMTS), located at Xinglong Station of NAOC, has a field of view up to 18 deg2. The TMTS has started to monitor the LAMOST sky areas since 2020, with the uninterrupted observations lasting for about 6 hours on average for each sky area and a cadence of about 1 minute. Here we introduce the data analysis and preliminary scientific results for the first-year observations, which covered 188 LAMOST plates (≈1970 deg2). These observations have generated over 4.9 million uninterrupted light curves, with at least 100 epochs for each of them. These light curves correspond to 4.26 million Gaia-DR2 sources, among which 285 thousand sources are found to have multi-epoch spectra from the LAMOST. By analysing these light curves with the Lomb-Scargle periodograms, we identify more than 3700 periodic variable star candidates with periods below ≈7.5 hours, primarily consisting of eclipsing binaries and δ Scuti stars. Those short-period binaries will provide important constraints on theories of binary evolution and possible sources for space gravitational wave experiments in the future. Moreover, we also identified 42 flare stars by searching rapidly-evolving signals in the light curves. The densely-sampled light curves from the TMTS allow us to better quantify the shapes and durations for these flares.

Chemo-kinematics of the Gaia RR Lyrae: the halo and the disc

ABSTRACT We present the results of a multicomponent kinematic model of a large sample of RR Lyrae detected by Gaia. By imposing a fourfold symmetry and employing Gaia proper motions, we are able to infer the behaviour of the velocity ellipsoid between ≈3 and ≈30 kpc from the centre of the Galaxy. We detect the presence of two distinct components: a dominant non-rotating halo-like population and a much smaller rotating disc-like population. We demonstrate that the halo RR Lyrae can be described as a superposition of an isotropic and radially biased parts. The radially biased portion of the halo is characterized by a high orbital anisotropy β ≈ 0.9 and contributes between 50 per cent and 80 per cent of the halo RR Lyrae at 5 &amp;lt; R(kpc)&amp;lt;25. In line with previous studies, we interpret this high-β component as the debris cloud of the ancient massive merger also known as the Gaia Sausage (GS) whose orbital extrema we constrain. The light-curve properties of the RR Lyrae support the kinematic decomposition: the GS stars are more metal-rich and boast higher fractions of Oosterhoff Type 1 and high-amplitude short period (HASP) variables compared to the isotropic halo component. The metallicity/HASP maps reveal that the inner 10 kpc of the halo is likely inhabited by the RR Lyrae born in situ. The mean azimuthal speed and the velocity dispersion of the disc RR Lyrae out to R ≈ 30 kpc are consistent with the behaviour of a young and metal-rich thin disc stellar population.

Near-infrared Census of RR Lyrae Variables in the Messier 3 Globular Cluster and the Period–Luminosity Relations

Abstract We present new near-infrared (NIR), JHK s , time-series observations of RR Lyrae variables in the Messier 3 (NGC 5272) globular cluster using the WIRCam instrument at the 3.6 m Canada–France–Hawaii Telescope. Our observations cover a sky area of ∼21′ × 21′ around the cluster center and provide an average of 20 epochs of homogeneous JHK s -band photometry. New homogeneous photometry is used to estimate robust mean magnitudes for 175 fundamental-mode (RRab), 47 overtone-mode (RRc), and 11 mixed-mode (RRd) variables. Our sample of 233 RR Lyrae variables is the largest thus far obtained in a single cluster with time-resolved, multiband NIR photometry. NIR-to-optical amplitude ratios for RR Lyrae in Messier 3 exhibit a systematic increase moving from RRc to short-period (P &lt; 0.6 day) and long-period (P ≳ 0.6 day) RRab variables. We derive JHK s -band period–luminosity relations for RRab, RRc, and the combined sample of variables. Absolute calibrations based on the theoretically predicted period–luminosity–metallicity relations for RR Lyrae stars yield a distance modulus, mag, to Messier 3. When anchored to trigonometric parallaxes for nearby RR Lyrae stars from the Hubble Space Telescope and the Gaia mission, our distance estimates are consistent with those resulting from the theoretical calibrations, albeit with relatively larger systematic uncertainties.

Variable stars in the field of intermediate-age open cluster NGC 559

ABSTRACT This work presents the first long-term photometric variability survey of the intermediate-age open cluster NGC 559. Time series V-band photometric observations on 40 nights taken over more than 3 yr with three different telescopes are analysed to search for variable stars in the cluster. We investigate the data for the periodicity analysis and reveal 70 variable stars including 67 periodic variables in the target field, all of them are newly discovered. The membership analysis of the periodic variables reveals that 30 of them belong to the cluster and remaining 37 are identified as field variables. Out of the 67 periodic variables, 48 are short-period (P &amp;lt; 1 d) variables and 19 are long-period (P &amp;gt; 1 d) variables. The variable stars have periodicity between 3 h to 41 d and their brightness ranges from V = 10.9 to 19.3 mag. The periodic variables belonging to the cluster are then classified into different variability types on the basis of observational properties such as shape of the light curves, periods, amplitudes, as well as their positions in the Hertzsprung–Russell (H–R) diagram. As a result, we identify 1 Algol type eclipsing binary, 1 possible blue straggler star, 3 slowly pulsating B type stars, 5 rotational variables, 11 non-pulsating variables, 2 FKCOM variables, and remaining 7 are characterized as miscellaneous variables. We also identify three eclipsing binary stars (EBs) belonging to the field star population. The phoebe package is used to analyse the light curve of all four EBs in order to determine the parameters of the binary systems such as masses, temperatures, and radii.

Physical properties of RR Lyrae variables in Galactic globular clusters and dwarf spheroidal galaxies

We propose a tool to distinguish between Galactic globular clusters (GCs) and dwarf galaxies based on physical properties of RR Lyrae (RRL) variables in the systems. Normally, these two systems show various distinctive properties; for instance, mass, size, and luminosity. Nevertheless, these properties overlap each other in recently discovered systems. The subsequent classification between GCs and dwarf spheroidal (dSph) galaxy is ambiguous. An RRL variable is a pulsating variable star with a short period of 0.2-1 days. It is relatively easy to observe the complete period of its light curve. The age of RRL stars is typically more than 10 Gyr; therefore, they can provide information of the host system at its early stage. In this study, we provide an extensive data collection of RRLs in 96 GCs, 23 dSphs, and 10 dwarf irregular (dIrr) galaxies. Based on public catalogs, we analyze various RRL properties such as Oosterhoff dichotomy, fraction of high amplitude short period (HASP) variables, metallicity, and specific frequency of RRLs (SRR). We examine correlations among all properties in an attempt to distinguish between GCs and dSphs.

Gaia Data Release 2

Aims. We describe the methods used and the analysis performed in the frame of the Gaia data processing activities to produce the Gaia Data Release 2 (DR2) sample candidates with short-timescale variability together with associated parameters. Methods. The Gaia DR2 sample of candidates with short-timescale variability results from the investigation of the first 22 months of Gaia G per-CCD, GBP, and GRP photometry for a subsample of sources at the Gaia faint end (G ~ 16.5−20 mag). For this first short-timescale variability search exploiting Gaia data, we limited ourselves to the case of suspected rapid periodic variability. Our study combines fast-variability detection through variogram analysis, a high-frequency search by means of least-squares periodograms, and an empirical selection based on the investigation of specific sources seen through the Gaia eyes (e.g., known variables or visually identified objects with peculiar features in their light curves). The progressive definition, improvement, and validation of this selection criterion also benefited from supplementary ground-based photometric monitoring of a few tens of preliminary candidates with short-timescale variability, performed at the Flemish Mercator telescope in La Palma (Canary Islands, Spain) between August and November 2017. Results. As part of Gaia DR2, we publish a list of 3018 candidates with short-timescale variability, spread throughout the sky, with a false-positive rate of up to 10–20% in the Magellanic Clouds, and a more significant but justifiable contamination from longer-period variables between 19% and 50%, depending on the area of the sky. Although its completeness is limited to about 0.05%, this first sample of Gaia short-timescale variables recovers some very interesting known short-period variables, such as post-common envelope binaries or cataclysmic variables, and brings to light some fascinating, newly discovered variable sources. In the perspective of future Gaia data releases, several improvements of the short-timescale variability processing are considered, by enhancing the existing variogram and period-search algorithms or by classifying the identified variability candidates. Nonetheless, the encouraging outcome of our Gaia DR2 analysis demonstrates the power of this mission for such fast-variability studies, and opens great perspectives for this domain of astrophysics.

Metal-poor Type II Cepheids with Periods Less Than Three Days

We have analyzed 10 high-resolution spectra of Type II Cepheids with periods less than 3 days. We find that they clearly separate into two groups: those with near or slightly below solar metallicities, and those with [Fe/H] between −1.5 and −2.0. While the former are usually called BL Her stars, we suggest that the latter be called UY Eri stars. The UY Eri subclass appears to be similar to the short period variables in globular clusters of the Galactic Halo. Globular clusters with [Fe/H] > −1.0 almost never have Type II Cepheids.

The OmegaWhite survey for short-period variable stars – III: follow-up photometric and spectroscopic observations

We present photometric and spectroscopic follow-up observations of short-period variables discovered in the OmegaWhite survey: a wide-field high-cadence g-band synoptic survey targeting the Galactic Plane. We have used fast photometry on the SAAO 1.0-m and 1.9-m telescopes to obtain light curves of 27 variables, and use these results to validate the period and amplitude estimates from the OmegaWhite processing pipeline. Furthermore, 57 sources (44 unique, 13 also with new light curves) were selected for spectroscopic follow-up using either the SAAO 1.9-m telescope or the Southern African Large Telescope. We find many of these variables have spectra which are consistent with being delta Scuti type pulsating stars. At higher amplitudes, we detect four possible pulsating white dwarf/subdwarf sources and an eclipsing cataclysmic variable. Due to their rarity, these targets are ideal candidates for detailed follow-up studies. From spectroscopy, we confirm the symbiotic binary star nature of two variables identified as such in the SIMBAD database. We also report what could possibly be the first detection of the `Bump Cepheid' phenomena in a delta Scuti star, with OW J175848.21-271653.7 showing a pronounced 22% amplitude dip lasting 3 minutes during each pulsational cycle peak. However, the precise nature of this target is still uncertain as it exhibits the spectral features of a B-type star.

The OmegaWhite Survey for short period variable stars – II. An overview of results from the first four years

OmegaWhite is a wide-field, high cadence, synoptic survey targeting fields in the southern Galactic plane, with the aim of discovering short period variable stars. Our strategy is to take a series of 39 s exposures in the g band of a 1 square degree of sky lasting 2 h using the OmegaCAM wide field imager on the VLT Survey Telescope (VST). We give an overview of the initial 4 years of data which covers 134 square degrees and includes 12.3 million light curves. As the fields overlap with the VLT Survey Telescope Halpha Photometric Survey of the Galactic plane and Bulge (VPHAS+), we currently have $ugriH\alpha$ photometry for ~1/3 of our fields. We find that a significant fraction of the light curves have been affected by the diffraction spikes of bright stars sweeping across stars within a few dozen of pixels over the two hour observing time interval due to the alt-az nature of the VST. We select candidate variable stars using a variety of variability statistics, followed by a manual verification stage. We present samples of several classes of short period variables, including: an ultra compact binary, a DQ white dwarf, a compact object with evidence of a 100 min rotation period, three CVs, one eclipsing binary with an 85 min period, a symbiotic binary which shows evidence of a 31 min photometric period, and a large sample of candidate delta Sct type stars including one with a 9.3 min period. Our overall goal is to cover 400 square degrees, and this study indicates we will find many more interesting short period variable stars as a result.

VARIABLE STARS IN THE FIELD OF THE HYDRA II ULTRA-FAINT DWARF GALAXY

ABSTRACT We report the discovery of one RR Lyrae star in the ultra-faint satellite galaxy Hydra II based on time series photometry in the g, r and i bands obtained with the Dark Energy Camera at Cerro Tololo Inter-American Observatory, Chile. The association of the RR Lyrae star discovered here with Hydra II is clear because is located at 42 ″ from the center of the dwarf, well within its half-light radius of 102 ″ . The RR Lyrae star has a mean magnitude of i = 21.30 ± 0.04 which is too faint to be a field halo star. This magnitude translates to a heliocentric distance of 151 ± 8 kpc for Hydra II; this value is ∼ 13 % larger than the estimate from the discovery paper based on the average magnitude of several blue horizontal branch star candidates. The new distance implies a slightly larger half-light radius of 76 − 10 + 12 pc and a brighter absolute magnitude of M V = − 5.1 ± 0.3 , which keeps this object within the realm of the dwarf galaxies. A comparison with other RR Lyrae stars in ultra-faint systems indicates similar pulsational properties among them, which are different to those found among halo field stars and those in the largest of the Milky Way satellites. We also report the discovery of 31 additional short period variables in the field of view (RR Lyrae, SX Phe, eclipsing binaries, and a likely anomalous cepheid) which are likely not related with Hydra II.

WEAK GALACTIC HALO-DWARF SPHEROIDAL CONNECTION FROM RR LYRAE STARS

We discuss the role that dwarf galaxies may have played in the formation of the Galactic halo (Halo) using RR Lyrae stars (RRL) as tracers of their ancient stellar component. The comparison is performed using two observables (periods, luminosity amplitudes) that are reddening and distance independent. Fundamental mode RRL in six dwarf spheroidals and eleven ultra faint dwarf galaxies (1,300) show a Gaussian period distribution well peaked around a mean period of <Pab>=0.610+-0.001 days (sigma=0.03). The Halo RRL (15,000) are characterized by a broader period distribution. The fundamental mode RRL in all the dwarf spheroidals apart from Sagittarius are completely lacking in High Amplitude Short Period (HASP) variables, defined as those having P< 0.48 days and Av> 0.75mag. Such variables are not uncommon in the Halo and among the globular clusters and massive dwarf irregulars. To further interpret this evidence, we considered eighteen globulars covering a broad range in metallicity (-2.3< [Fe/H]< -1.1) and hosting more than 35 RRL each. The metallicity turns out to be the main parameter, since only globulars more metal--rich than [Fe/H] -1.5 host RRL in the HASP region. This finding suggests that dSphs similar to the surviving ones do not appear to be the major building-blocks of the Halo. Leading physical arguments suggest an extreme upper limit of 50% to their contribution. On the other hand, massive dwarfs hosting an old population with a broad metallicity distribution (Large Magellanic Cloud, Sagittarius) may have played a primary role in the formation of the Halo.

FIRST DETECTION OF THE WHITE DWARF COOLING SEQUENCE OF THE GALACTIC BULGE

We present Hubble Space Telescope data of the low-reddening Sagittarius window in the Galactic bulge. The Sagittarius Window Eclipsing Extrasolar Planet Search field (3'x3'), together with three more Advanced Camera for Surveys and eight Wide Field Camera 3 fields, were observed in the F606W and F814W filters, approximately every two weeks for two years, with the principal aim of detecting a hidden population of isolated black holes and neutron stars through astrometric microlensing. Proper motions were measured with an accuracy of ~0.1 mas/yr (~4 km/s) at F606W~25.5 mag, and better than ~0.5 mas/yr (20 km/s) at F606W~28 mag, in both axes. Proper-motion measurements allowed us to separate disk and bulge stars and obtain a clean bulge color-magnitude diagram. We then identified for the first time a white dwarf (WD) cooling sequence in the Galactic bulge, together with a dozen candidate extreme horizontal branch stars. The comparison between theory and observations shows that a substantial fraction of the WDs (30%) are systematically redder than the cooling tracks for CO-core H-rich and He-rich envelope WDs. This evidence would suggest the presence of a significant number of low-mass WDs and WD - main sequence binaries in the bulge. This hypothesis is further supported by the finding of two dwarf novae in outburst, two short-period (P < 1 d) ellipsoidal variables, and a few candidate cataclysmic variables in the same field.