We identify SDSSJ 121010.1+334722.9 as an eclipsing post-common-envelope binary , with an orbital period of \mbox { $P _ { \mathrm { orb } } $ } = 2.988 \mathrm { 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 GALEX near-ultraviolet flux , yields a white dwarf temperature of \mbox { $T _ { \mathrm { eff,WD } } $ } = 6000 \pm 200 K and a metallicity value of \log [ \mathrm { Z / H } ] = -2.0 \pm 0.3 .
The Na I \lambda \lambda 8183.27,8194.81 absorption doublet is used to measure the radial velocity of the secondary star , \mbox { $K _ { \mathrm { sec } } $ } = 251.7 \pm 2.0 \mbox { $ \mathrm { km s ^ { -1 } } $ } and Fe I absorption lines in the blue part of the spectrum provide the radial velocity of the white dwarf , \mbox { $K _ { \mathrm { WD } } $ } = 95.3 \pm 2.1 \mbox { $ \mathrm { km s ^ { -1 } } $ } , yielding a mass ratio of q = 0.379 \pm 0.009 .
Light curve model fitting , using the Markov Chain Monte Carlo ( MCMC ) method , gives the inclination angle as i = ( 79.05 ^ { \circ } - 79.36 ^ { \circ } ) \pm 0.15 ^ { \circ } , and the stellar masses as \mbox { $M _ { \mathrm { WD } } $ } = 0.415 \pm 0.010 \mbox { $ \mathrm { M } _ { \odot } $ } and \mbox { $M _ { \mathrm { sec } } $ } = 0.158 \pm 0.006 \mbox { $ \mathrm { M } _ { \odot } $ } .
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 \mbox { $R _ { \mathrm { WD } } $ } = ( 0.0157 - 0.0161 ) \pm 0.0003 \mbox { $ \mathrm { % R } _ { \odot } $ } and the volume-averaged radius of the tidally distorted secondary is \mbox { $R _ { \mathrm { sec,vol . aver . } } $ } = ( 0.197 - 0.203 ) \pm 0.003 \mbox { $% \mathrm { R } _ { \odot } $ } .
The white dwarf in SDSS1210 is a very strong He-core candidate .