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From optical photometry we show that SDSSJ121258.25-012310.1 is a neweclipsing, post common-envelope binary with an orbital period of 8.06 hours andan eclipse length of 23 minutes. We observed the object over 11 nights indifferent bands and determined the ephemeris of the eclipse to HJD_mid =2454104. 7086(2) + 0.3358706(5) x E, where numbers in parenthesis indicate theuncertainties in the last digit. The depth of the eclipse is 2.85 +/- 0.17 magin the V band, 1.82 +/- 0.08 mag in the R band and 0.52 +/- 0.02 mag in the Iband. From spectroscopic observations we measured the semi-amplitude of theradial velocity K_2 = 181 +/- 3 km/s for the secondary star. The stellar andbinary parameters of the system were constrained from a) fitting the SDSScomposite spectrum of the binary, b) using a K-band luminosty-mass relation forthe secondary star, and c) from detailed analyses of the eclipse light curve.The white dwarf has an effective temperature of 17700 +/- 300 K, and itssurface gravity is logg =7.53 +/- 0.2. We estimate that the spectral type ofthe 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 = 0.26-0.29Msun, while the mass of the white dwarf is most likely Mwd = 0.46-0.48 Msun.  From an empirical mass-radius relation we estimate the radius of the reddwarf to be in the range 0.28-0.31 Rsun, whereas we get Rwd = 0.016-0.018 Rsunfrom a theoretical mass-radius realation. Finally we discuss the spectralenergy distribution and the likely evolutionary state of SDSS1212-0123.

Post common envelope binaries from SDSS. IV: SDSSJ121258.25-012310.1, a new eclipsing system