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 ) mins , with no evidence for other periods in its light curves .
This period has remained constant over the last two years 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 ionised carbon , a broad He I \lambda 4471 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 } = 15430 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 .