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Published April 20, 2013 | Published
Journal Article Open

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_★ = 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.

Additional Information

© 2013 American Astronomical Society. Received 2013 January 2; accepted 2013 March 7; published 2013 April 2. We would like to thank Eric Agol, Jean-Michel Désert, Dong Lai, Dylan Morgan, Tony Piro and Andrew West for their insightful communications about this system. We would like to thank the anonymous referee for the thoughtful and constructive comments. We would like to thank the staff at Palomar Observatory for providing support during our many observation runs, including Bruce Baker, Mike Doyle, Jamey Eriksen, Carolyn Heffner, John Henning, Steven Kunsman, Dan McKenna, Jean Mueller, Kajsa Peffer, Kevin Rykoski, and Greg van Idsinga. J. Becker would like to thank Mr. and Mrs. Kenneth Adelman for providing funding for her 2012 Alain Porter Memorial SURF Fellowship. A portion of this work was supported by the National Science Foundation under grant No. AST-1203023. The Robo-AO system is supported by collaborating partner institutions, the California Institute of Technology and the Inter-University Centre for Astronomy and Astrophysics, by the National Science Foundation under grant Nos. AST-0906060 and AST-0960343, by a grant from the Mt. Cuba Astronomical Foundation and by a gift from Samuel Oschin. This paper includes data collected by the Kepler mission. Funding for the Kepler mission is provided by the NASA Science Mission directorate. Some of the data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has within the indigenous Hawaiian community. Facilities: GALEX, Keck:I (HIRES), Keck:II (ESI), Kepler, PO:1.5m (Robo-AO), Hale (TripleSpec, WIRC)

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