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 hours and an eclipse length of 23 minutes .
We observed the object over 11 nights in different bands and determined the ephemeris of the eclipse to HJD _ { \mathrm { mid } } = 2454104.7086 ( 2 ) +0.3358706 ( 5 ) \times \mathrm { E } , where numbers in parenthesis indicate the uncertainties in the last digit .
The depth of the eclipse is 2.85 \pm 0.17 mag in the V band , 1.82 \pm 0.08 mag in the R band and 0.52 \pm 0.02 mag in the I band .
From spectroscopic observations we measured the semi-amplitude of the radial velocity K _ { 2 } = 181 \pm 3 km/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 \pm 300 K , and its surface gravity is \mbox { $ \log g$ } = 7.53 \pm 0.2 .
We estimate that the spectral type of the red dwarf is \mathrm { M 4 } \pm 1 and the distance to the system is 230 \pm 20 parsec .
The mass of the secondary star is estimated to be in the range \mbox { $M _ { \mathrm { sec } } $ } = 0.26 - 0.29 M _ { \odot } , while the mass of the white dwarf is most likely \mbox { $M _ { \mathrm { wd } } $ } = 0.46 - 0.48 M _ { \odot } .
From an empirical mass-radius relation we estimate the radius of the red dwarf to be in the range 0.28 - 0.31 R _ { \odot } , whereas we get \mbox { $R _ { \mathrm { wd } } $ } = 0.016 - 0.018 R _ { \odot } from a theoretical mass-radius realation .
Finally we discuss the spectral energy distribution and the likely evolutionary state of SDSS1212–0123 .