  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 4471 A 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.
