2012MNRAS.419..817P

We identify SDSS J121010.1+334722.9 as an eclipsing post-common-envelope binary, with an orbital period of P_orb = 2.988 h, containing a very cool, low-mass, DAZ white dwarf and a low-mass main-sequence star of spectral type M5. A model atmosphere analysis of the metal absorption lines detected in the blue part of the optical spectrum, along with the Galaxy Evolution Explorer near-ultraviolet flux, yields a white dwarf temperature of T_eff,WD = 200 K and a metallicity value of log [Z/H] =-2.0±0.3. The Na I λλ8183.27, 8194.81 absorption doublet is used to measure the radial velocity of the secondary star, K_sec = 251.7 ±2.0 km/s, and Fe I absorption lines in the blue part of the spectrum provide the radial velocity of the white dwarf, K_WD = 95.3 ±2.1 km/s, yielding a mass ratio of q = 0.379±0.009. Light-curve model fitting, using the Markov chain Monte Carlo method, gives the inclination angle as i = (79°.5-79°.36)±0°.15, and the stellar masses as M_WD = 0.415 ± 0.010M_☉ and M_sec = 0.158 ± 0.0006M_☉. Systematic uncertainties in the absolute calibration of the photometric data influence the determination of the stellar radii. The radius of the white dwarf is found to be R_WD = (0.0157-0.0161) ± 0.0003 R_☉ and the volume-averaged radius of the tidally distorted secondary is R_{sec,vol.aver.} = (0.197-0.203) ± 0.003 R_☉. The white dwarf in SDSS J121010.1+334722.9 is a very strong He-core candidate.