Attributing the O’Connell Effect in Contact Binaries to a Cooling Mass-transfer Stream

Abstract Contact binaries are very short-period systems that are continuously interacting by transferring mass and energy. Obtaining large, statistical samples of contact binaries from photometric surveys can put valuable constraints on the various processes involved in their evolution. Modeling those systems, however, presents some challenges. In some contact-binary light curves, the O’Connell effect is visible, where the maxima at both quarter phases are unequal. In the literature, this effect is typically attributed to magnetic spots on the surface of the binary. In this work, we model contact-binary surfaces using PHOEBE and include a parametric prescription for a lateral mass- and energy-transfer stream that travels from the hotter primary to the cooler secondary. We allow this stream to have a variable heat capacity. We fit a system from the Kepler sample with a strong O’Connell effect and show that a low-heat capacity stream can explain the unequal maxima. This suggests that, in such systems, surface flows can play a significant role in transferring heat between components. Our methods can be used on larger samples of contact binaries from the Optical Gravitational Lensing Experiment, Kepler, or TESS to advance our understanding of contact binary structure and evolution.