The Binary–Binary Hierarchical System XY Leo: A Laboratory for Stellar Activity and Concealed Companions

Abstract The hierarchical multiple system XY Leo, despite nearly 90 yr of observations, remains enigmatic. It offers a unique testbed for close binary evolution, involving processes like mass transfer, angular momentum loss, and the von Zeipel–Lidov–Kozai mechanism. Previously identified as a quadruple system, XY Leo shows long-term orbital period modulations. Our new ground-based and Transiting Exoplanet Survey Satellite data suggest this may stem from either magnetic cycles or the influence of an unseen companion. While the latter remains speculative, both scenarios are discussed within a unified framework. Using all available photometric and spectroscopic data, we derived ultraprecise physical parameters for the contact binary XY Leo A as M A1 = 0.629 ± 0.009 M ⊙ , M A2 = 0.865 ± 0.012 M ⊙ , R A1 = 0.739 ± 0.007 R ⊙ , R A2 = 0.855 ± 0.008 R ⊙ , L A1 = 0.271 ± 0.026 L ⊙ , and L A2 = 0.288 ± 0.030 L ⊙ and orbital separation a A = 2.078 ± 0.010 R ⊙ based on simultaneous solutions of light and radial velocity curves. The detached binary subsystem XY Leo B is confirmed to be on a wide ∼20 yr orbit around the contact system. A second ∼23 yr modulation is also detected, which may stem from either stellar magnetic activity or an additional, unseen companion. After removing both trends, a coherent residual modulation with a characteristic timescale of 14.2 ± 0.8 yr remains in the O – C diagram, consistent with a magnetic activity cycle of Applegate type. We modeled XY Leo A with the Cambridge STARS (EV/TWIN) code under nonconservative evolution, finding strong agreement between the tracks and observed parameters—highlighting the system’s value for testing multiple-star evolution.