Characterizing the Cool KOIs. V. KOI-256: A Mutually Eclipsing Post-common Envelope Binary

We report that Kepler Object of Interest 256 (KOI-256) is a mutually eclipsing post-common envelope binary (ePCEB), consisting of a cool white dwarf (M _sstarf = 0.592 ± 0.089 M _⊙, R _sstarf = 0.01345 ± 0.00091 R _⊙, T _eff = 7100 ± 700 K) and an active M3 dwarf (M _sstarf = 0.51 ± 0.16 M _⊙, R _sstarf = 0.540 ± 0.014 R _⊙, T _eff = 3450 ± 50 K) with an orbital period of 1.37865 ± 0.00001 days. KOI-256 is listed as hosting a transiting planet-candidate by Borucki et al. and Batalha et al.; here we report that the planet-candidate transit signal is in fact the occultation of a white dwarf as it passes behind the M dwarf. We combine publicly-available long- and short-cadence Kepler light curves with ground-based measurements to robustly determine the system parameters. The occultation events are readily apparent in the Kepler light curve, as is spin-orbit synchronization of the M dwarf, and we detect the transit of the white dwarf in front of the M dwarf halfway between the occultation events. The size of the white dwarf with respect to the Einstein ring during transit (R _Ein = 0.00473 ± 0.00055 R _⊙) causes the transit depth to be shallower than expected from pure geometry due to gravitational lensing. KOI-256 is an old, long-period ePCEB and serves as a benchmark object for studying the evolution of binary star systems as well as white dwarfs themselves, thanks largely to the availability of near-continuous, ultra-precise Kepler photometry.