Mass-transfer properties of overcontact systems in the Kepler eclipsing binary catalog

We selected mass-transferring binary candidates from the catalog of Kepler eclipsing binary stars and investigated the dependence of the mass-transfer rate on several astrophysical quantities, including orbital period, semi-major axis, mass ratio, fill-out factor, temperature, and mass. We selected these candidates using $O-C$ diagrams and calculated their mass-transfer rates. Primary masses were obtained from the mass--temperature relation, and the temperatures of the component stars were extracted from a catalog of temperatures for Kepler eclipsing binary stars. The mass-transfer rates of overcontact systems have associations with astrophysical quantities that seem to differ from those of semi-detached or detached systems. These associations indicate that mass exchange from more- to less-massive components (from less- to more-massive components) generally becomes rapid (slow) as the mass exchange evolves. However, for mass exchange from more- to less-massive components, this tendency is not reasonable for binaries with a short period ($P<0.4$ d) and low mass ($M_1<1.2$ M$_\odot$) because the correlations of these binaries are opposite to those of binaries with long period and high mass. These different correlations likely arise from differences in correlation between subtypes of W UMa systems (i.e., W- and A-types). Alternatively, when mass exchange from more- to less-massive components reoccurs after a mass-ratio reversal, its properties may differ from those of the first mass exchange.