Post-common envelope binaries from SDSS - XV. Accurate stellar parameters for a cool 0.4 M⊙ white dwarf and a 0.16 M⊙ M dwarf in a 3 h eclipsing binary

We identify SDSS J121010.1+334722.9 as an eclipsing post-common-envelope binary, with an orbital period of Graphic, 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 Graphic 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, Graphic, and Fe I absorption lines in the blue part of the spectrum provide the radial velocity of the white dwarf, Graphic, 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°.05–79°.36) ± 0°.15, and the stellar masses as Graphic and Graphic. 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 Graphic and the volume-averaged radius of the tidally distorted secondary is Graphic. The white dwarf in SDSS J121010.1+334722.9 is a very strong He-core candidate.