We report the discovery and analysis of PTF1 J085713+331843, a new eclipsing post common-envelope detached white-dwarf red-dwarf binary with a 2.5h orbital period discovered by the Palomar Transient Factory. ULTRACAM multicolour photometry over multiple orbital periods reveals a light curve with a deep flat-bottomed primary eclipse and a strong reflection effect. Phase-resolved spectroscopy shows broad Balmer absorption lines from the DA white dwarf and phase-dependent Balmer emission lines originating on the irradiated side of the red dwarf. The temperature of the DA white dwarf is $T_mathrm{WD} = 25700 pm 400,$K and the spectral type of the red dwarf is M3-5. A combined modelling of the light curve and the radial velocity variations results in a white dwarf mass of $M_mathrm{WD} = 0.61^{+0.18}_{-0.17}, mathrm{M_{odot}}$ and radius of $R_mathrm{WD} = 0.0175^{+0.0012}_{-0.0011}, mathrm{R_{odot}}$, and a red dwarf mass and radius of $M_mathrm{RD} = 0.19^{+0.10}_{-0.08}, mathrm{M_{odot}}$ and $R_mathrm{RD} = 0.24^{+0.04}_{-0.04}, mathrm{R_{odot}}$. The system is either a detached cataclysmic variable or has emerged like from the common envelope phase at nearly its current orbital period. In $sim70,$Myr, this system will become a cataclysmic variable in the period gap.
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