The OmegaWhite survey for short-period variable stars - I. Overview and first results

We present the goals, strategy and first results of the OmegaWhite survey: a wide-field 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 deg(2) along the Galactic plane reaching a depth of g = 21.5 mag (10 sigma), using OmegaCam on the VLT Survey Telescope (VST). The fields are selected to overlap with surveys such as the Galactic Bulge Survey and the VST Photometric Ha Survey of the Southern Galactic Plane for multiband colour information. Each field is observed using 38 exposures of 39 s each, with a median cadence of similar to 2.7 min for a total duration of two hours. Within an initial 26 deg(2), 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 delta Sct pulsating stars.