PTF1 J085713+331843, a new post-common-envelope binary in the orbital period gap of cataclysmic variables

We report the discovery and analysis of PTF1 J085713+331843, a new eclipsing post-common-envelope detached white-dwarf–red-dwarf binary with a 2.5 h 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_(WD) = 25 700 ± 400 K and the spectral type of the red dwarf is M3–M5. A combined modelling of the light curve and the radial velocity variations results in a white dwarf mass of M_(WD)=0.61^(+0.18)_(−0.17)M⊙ and radius of R_(WD)=0.0175^(+0.0012)_(−0.0011)R⊙, and a red dwarf mass and radius of M_(RD)=0.19^(+0.10)_(−0.08)M⊙ and R_(RD)=0.24^(+0.04)_(−0.04)R⊙. The system is either a detached cataclysmic variable or has emerged like from the common-envelope phase at nearly its current orbital period. In ∼70 Myr, this system will become a cataclysmic variable in the period gap