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 inline image, 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 inline image 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, inline image, and Fe i absorption lines in the blue part of the spectrum provide the radial velocity of the white dwarf, inline image, 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 = (79bsl0006405–79bsl0006436) ± 0bsl0006415, and the stellar masses as inline image and inline image. 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 inline image and the volume-averaged radius of the tidally distorted secondary is inline image. The white dwarf in SDSS J121010.1+334722.9 is a very strong He-core candidate.