CW Cas: A Solar-type Contact Binary System with an Unseen Third Companion in a Hierarchical Quadruple System

Abstract We present a comprehensive multiband photometric and spectroscopic study of the G-type binary CW Cas. Analysis of its double-lined spectroscopic radial velocity curves yields a reliable mass ratio of q = 1.88(9). By combining modeling of the radial velocity curves with BVR c I c and TESS light curves, we found that CW Cas is a W-subtype shallow contact binary with a fill-out factor of 15%. The components have masses of 0.98(6) M ⊙ and 0.52(4) M ⊙ , separated by 2.25(5) R ⊙ . A notable asymmetry in the maxima of the light curves was detected and explained by a dark spot located on the surface of at least one component. Comparison of light curves from different years revealed that these dark spot activities exhibit cyclic variations with an approximate period of 349 days. Orbital period analysis via its O − C diagram spanning 125 yr shows a long-term decrease superimposed with a periodic oscillation caused by the light-travel time effect due to a third body. This tertiary component has an orbital period of P 3 = 99.4(6) yr and a minimal mass of M 3 = 0.91(1) M ⊙ . The absence of detectable signatures for this massive object in either the spectroscopic or photometric datasets implies it should be a compact object. Furthermore, a visual companion was identified based on Gaia Data Release 3 astrometric data, suggesting that CW Cas is part of a hierarchical quadruple system. As such, CW Cas represents a valuable laboratory for probing a 2 + 1 + 1 hierarchical multiple system hosting a compact object.

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