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* = 0.592 ± 0.089 M, R = 0.01345 ± 0.00091 R , T eff = 7100 ± 700 K) and an active M3 dwarf (M* = 0.51 ± 0.16 M , R* = 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. © 2013. The American Astronomical Society. All rights reserved..
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Muirhead, P., Vanderburg, A., Shporer, A., Becker, J., Swift, J., Lloyd, J., Fuller, J., Zhao, M., Hinkley, S., Pineda, J., Bottom, M., Howard, A., Von Braun, K., Boyajian, T., Law, N., Baranec, C., Riddle, R., Ramaprakash, A., Tendulkar, S., Bui, K., Burse, M., Chordia, P., Das, H., Dekany, R., Punnadi, S., & Johnson, J. (2013). Characterizing the cool kois. V. koi-256: A mutually eclipsing post-common envelope binary. Astrophysical Journal, 767 (2) https://doi.org/10.1088/0004-637X/767/2/111