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

Abstract

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 $M_{\mathrm{WD}}=0.61_{-0.17}^{+0.18}{\mathrm{M}}_{\odot }$ and radius of $R_{\mathrm{WD}}=0.0175_{-0.0011}^{+0.0012}{\mathrm{R}}_{\odot }$, and a red dwarf mass and radius of $M_{\mathrm{RD}}=0.19_{-0.08}^{+0.10}{\mathrm{M}}_{\odot }$ 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 $\sim 70$Myr, this system will become a cataclysmic variable in the period gap.