We present the goals, strategy and first results of the OmegaWhite survey: a widefield high-cadence g-band synoptic survey which aims to unveil the Galactic population of short-period variable stars (with periods < 80 min), including ultracompact binary star systems and stellar pulsators. The ultimate goal of OmegaWhite is to cover 400 square degrees along the Galactic Plane reaching a depth of g = 21.5 mag (10σ), using OmegaCam on the VLT Survey Telescope (VST). The fields are selected to overlap with surveys such as the Galactic Bulge Survey (GBS) and the VST Photometric Hα Survey of the Southern Galactic Plane (VPHAS+) for multi-band colour information. Each field is observed using 38 exposures of 39 s each, with a median cadence of ∼2.7 min for a total duration of two hours. Within an initial 26 square degrees, we have extracted the light curves of 1.6 million stars, and have identified 613 variable candidates which satisfy our selection criteria. Furthermore, we present the light curves and statistical properties of 20 sources which have the highest-likelihood of being variable stars. One of these candidates exhibits the colours and light curve properties typically associated with ultracompact AM CVn binaries, although its spectrum exhibits weak Balmer absorption lines and is thus not likely to be such a binary system. We also present follow-up spectroscopy of five other variable candidates, which identifies them as likely low-amplitude δ Sct pulsating stars.

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 Hα Photometric Survey of the Galactic plane and Bulge (VPHAS+), we currently have ugriHα 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 δ 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.

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We report the discovery of the ultracompact hot subdwarf (sdOB) binary OW J074106.0-294811.0 with an orbital period of P 44.66279 1.16 10 orb 4 = ´-minutes, making it the most compact hot subdwarf binary known. Spectroscopic observations using the VLT, Gemini and Keck telescopes revealed a He-sdOB primary with an intermediate helium abundance, T eff = 39 400 500  K and g log = 5.74±0.09. High signal-to-noise ratio light curves show strong ellipsoidal modulation resulting in a derived sdOB mass M 0.23 0.12The mass ratio was found to be q M M 0.32 0.10 sdOB WD = =  . The derived mass for the He-sdOB is inconsistent with the canonical mass for hot subdwarfs of 0.47 » M  . To put constraints on the structure and evolutionary history of the sdOB star we compared the derived T eff , g log , and sdOB mass to evolutionary tracks of helium stars and helium white dwarfs calculated with Modules for Experiments in Stellar Astrophysics (MESA). We find that the best-fitting model is a helium white dwarf with a mass of 0.320 M  , which left the common envelope 1.1 Myr » ago, which is consistent with the observations. As a helium white dwarf with a massive white dwarf companion, the object will reach contact in 17.6 Myr at an orbital period of 5 minutes. Depending on the spin-orbit synchronization timescale the object will either merge to form an R CrB star or end up as a stably accreting AM CVn-type system with a helium white dwarf donor.

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