The OmegaWhite Survey for short-period variable stars – IV. Discovery of the warm DQ white dwarf OW J175358.85−310728.9

We present the discovery and follow-up observations of the second known variable warm DQ white dwarf OW J175358.85−310728.9 (OW J1753−3107). OW J1753−3107 is the brightest of any of the currently known warm or hot DQ and was discovered in the OmegaWhite Survey as exhibiting optical variations on a period of 35.5452 (2) min, with no evidence for other periods in its light curves. This period has remained constant over the last 2 yr and a single-period sinusoidal model provides a good fit for all follow-up light curves. The spectrum consists of a very blue continuum with strong absorption lines of neutral and ionized carbon, a broad He I λ4471 line and possibly weaker hydrogen lines. The C I lines are Zeeman split, and indicate the presence of a strong magnetic field. Using spectral Paschen–Back model descriptions, we determine that OW J1753−3107 exhibits the following physical parameters: T_(eff)= 15 430 K, log (g) = 9.0, log (N(C)/N(He)) = −1.2 and the mean magnetic field strength is B_z =2.1 MG. This relatively low temperature and carbon abundance (compared to the expected properties of hot DQs) is similar to that seen in the other warm DQ SDSS J1036+6522. Although OW J1753−3107 appears to be a twin of SDSS J1036+6522, it exhibits a modulation on a period slightly longer than the dominant period in SDSS J1036+6522 and has a higher carbon abundance. The source of variations is uncertain, but they are believed to originate from the rotation of the magnetic white dwarf.