K. Tran, A. Levine, S. Rappaport, T. Borkovits, Sz. Csizmadia, B. Kalomeni
We report on a study of eclipse times in contact binary systems, using long-cadence lightcurves in the Kepler archive. 'Observed minus calculated' (O-C) curves were produced for both the primary and secondary eclipses of the contact binaries. These curves are distinct for their random-walk like variations, with typical amplitudes of +/- 200-300 seconds, quasi-periodicities, and anticorrelations between the curves of the primary and secondary eclipses. These phenomena cannot be explained by a model involving mass transfer, which among other things requires implausibly high rates of ~0.01 M_sun/year. We show that the anticorrelated behavior, the amplitude of the O-C delays, and the overall random-walk like behavior can be explained by the presence of a starspot that is continuously visible around the orbit and slowly changes its longitude on timescales of weeks to months. The quasi-periods of ~50-200 days observed in the O-C curves suggest values for k, the coefficient of the latitude dependence of the stellar differential rotation, of ~0.003-0.013.
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http://arxiv.org/abs/1305.4639
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