We present the discovery of photometric variability in the DQ white dwarf SDSS J103655.39+652252.2 ( SDSS J1036+6522 ) .
Time-series photometry reveals a coherent monoperiodic modulation at a period of 1115.64751 ( 67 ) s with an amplitude of 0.442 \% \pm 0.024 \% ; no other periodic modulations are observed with amplitudes \gtrsim 0.13 \% .
The period , amplitude , and phase of this modulation are constant within errors over 16 months .
The spectrum of SDSS J1036+6522 shows magnetic splitting of carbon lines , and we use Paschen-Back formalism to develop a grid of model atmospheres for mixed carbon and helium atmospheres .
Our models , while reliant on several simplistic assumptions , nevertheless match the major spectral and photometric properties of the star with a self-consistent set of parameters : T _ { eff } \approx 15 , 500 K , \log g \approx 9 , \log ( \mathrm { C } / \mathrm { He } ) = -1.0 , and a mean magnetic field strength of 3.0 \pm 0.2 MG .
The temperature and abundances strongly suggest that SDSS J1036+6522 is a transition object between the hot , carbon-dominated DQs and the cool , He-dominated DQs .
The variability of SDSS J1036+6522 has characteristics similar to those of the variable hot carbon-atmosphere white dwarfs ( DQVs ) , however , its temperature is significantly cooler .
The pulse profile of SDSS J1036+6522 is nearly sinusoidal , in contrast with the significantly asymmetric pulse shapes of the known magnetic DQVs .
If the variability in SDSS J1036+6522 is due to the same mechanism as other DQVs , then the pulse shape is not a definitive diagnostic on the absence of a strong magnetic field in DQVs .
It remains unclear whether the root cause of the variability in SDSS J1036+6522 and the other hot DQVs is the same .