CW Cas: A solar-type contact binary system with an unseen third companion in a hierarchical quadruple system

We present a comprehensive multiband photometric and spectroscopic study of the G-type binary CW Cas whose parameters have not been well determined. Our double-lined spectroscopic radial velocity curve of this system yields a reliable mass ratio of $q = 1.88(9)$. By combining $BVR_{c}I_{c}$ bands, TESS light curves and radial velocity 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 $0.98(6)M_{\odot}$ and $0.52(4)M_{\odot}$, separated by $2.25(5)R_{\odot}$. 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 1250 days. Orbital period analysis via O-C diagram spanning 125 years shows a long-term decrease superimposed with periodic oscillation caused by the light-travel-time effect (LTTE) due to a third body. This tertiary component has an orbital period of $P_{3} = 99.4(6)$ years and a minimal mass of $M_{3}=0.91(1)M_{\odot}$. The absence of detectable signatures for this massive object in either spectroscopic or photometric datasets implies it must be a compact object such as a white dwarf or neutron star. Furthermore, a visual companion was identified based on Gaia DR3 astrometric data, suggesting that CW Cas is part of a hierarchical quadruple system. As such, CW Cas represents a valuable laboratory for probing 2+1+1 hierarchical multiple system hosting compact object.