Period Analysis, Roche Modeling and Absolute Parameters for AU Ser, an Overcontact Binary System

Abstract

CCD photometric data collected at UnderOak Observatory (UO) and Desert Bloom Observatory (DBO) in three bandpasses (B, V and I_C) produced 10 new times of minimum for AU Ser which were used to revise the linear ephemeris. These results captured in 2011 and 2018 reinforced a longstanding observation that the shape of the light curve from this W UMa binary system (P=0.386497 d) is highly variable. Significantly skewed peaks and differences at maximum light were detected during quadrature which could only be simulated during Roche modeling by positioning a hot spot on the secondary star close to the neck between both constituents. Historically this system has been variously classified as an F8, G5 and K0 system; however, this study supports more recent reports that AU Ser is best described as spectral type K1V-K2V. A fresh assessment of eclipse time residuals over the past 80 years has provided additional insight regarding cyclical changes in orbital period experienced by this interesting variable star.