BS Cassiopeiae: A Contact Binary with Starspot-driven Variability and Evidence for Additional Components

Abstract

We present a comprehensive photometric and spectroscopic analysis of the contact eclipsing binary BS Cas, based on 22 yr multisite light curves and 2 yr high-resolution spectra. The long-term photometry shows pronounced year-to-year changes in the light-curve shape, including eclipse-depth reversals between the primary and secondary minima and variations in the O’Connell effect. The first double-lined radial-velocity (RV) curves of BS Cas are derived from the spectra using broadening-function (BF) profiles. Sharp peaks absent from the 2018 BFs appear in the 2020 BFs, suggesting an additional component along the same line of sight as BS Cas. Simultaneous modeling of the BV R light curves and the RVs shows that BS Cas is a contact binary in which the more massive component is cooler than its less massive companion, with a mass ratio of 2.515 and a fill-out factor of 32.3%. Multiepoch modeling suggests that the long-term photometric variations arise from evolving starspots and changes in the third-light contribution. The eclipse-timing variation diagram confirms a secular orbital period decrease at a rate of −2.305 × 10−7 days yr−1 consistent with angular momentum loss and mass transfer, along with an 18.376 yr modulation interpreted as a light-travel-time effect from a third body. The TESS timings reveal anticorrelated variations between the primary and secondary minima that appear to result from differential rotation of high-latitude starspots. These results suggest that BS Cas is a physically complex contact binary in which long-term photometric and eclipse-timing variations reflect multiple processes, including magnetic activity and the influence of a third body.