2009A&A...495..561N

From optical photometry we show that SDSSJ121258.25-012310.1 is a new eclipsing, post common-envelope binary with an orbital period of 8.06 h and an eclipse length of 23 min. We observed the object over 11 nights in different bands and determined the ephemeris of the eclipse to HJD_mid=2454104.7086(2)+0.3358706(5)xE, where numbers in parenthesis indicate the uncertainties in the last digit. The depth of the eclipse is 2.85±0.17mag in the V band, 1.82±0.08mag in the R band and 0.52±0.02mag in the I band. From spectroscopic observations we measured the semi-amplitude of the radial velocity K₂=181±3km/s for the secondary star. The stellar and binary parameters of the system were constrained from a) fitting the SDSS composite spectrum of the binary, b) using a K-band luminosty-mass relation for the secondary star, and c) from detailed analyses of the eclipse light curve. The white dwarf has an effective temperature of 17700±300K, and its surface gravity is logg=7.53±0.2. We estimate that the spectral type of the red dwarf is M4±1 and the distance to the system is 230±20parsec. The mass of the secondary star is estimated to be in the range M_sec=0.26-0.29M_☉, while the mass of the white dwarf is most likely M_wd=0.46-0.48M_☉. From an empirical mass-radius relation we estimate the radius of the red dwarf to be in the range 0.28-0.31R_☉, whereas we get R_wd=0.016-0.018R_☉ from a theoretical mass-radius realation. Finally we discuss the spectral energy distribution and the likely evolutionary state of SDSS1212-0123.