Abstract

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 HJDmid, where numbers in parenthesis indicate the uncertainties in the last digit. The depth of the eclipse is 2.85±0.17 mag in the V band, 1.82±0.08 mag in the R band and 0.52±0.02 mag in the I band. From spectroscopic observations we measured the semi-amplitude of the radial velocity K2 = 181 ±3kms-1 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 17 700±300 K, and its surface gravity is log g = 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±20 parsec. The mass of the secondary star is estimated to be in the range Msec = 0.26-0.29 M?, while the mass of the white dwarf is most likely Mwd = 0.46-0.48 M?. From an empirical mass-radius relation we estimate the radius of the red dwarf to be in the range 0.28-0.31 R?, whereas we get Rwd = 0.016-0.018 R ? from a theoretical mass-radius realation. Finally we discuss the spectral energy distribution and the likely evolutionary state of SDSS1212-0123. © 2009 ESO.