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 such as mass transfer, angular momentum loss, and the von Zeipel--Kozai--Lidov (ZKL) 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 that these 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 $M_{\rm A1}=0.629\pm0.009\,M_{\odot}$, $M_{\rm A2}=0.865\pm0.012\,M_{\odot}$, $R_{\rm A1}=0.739\pm0.007\,R_{\odot}$, $R_{\rm A2}=0.855\pm0.008\,R_{\odot}$, $L_{\rm A1}=0.271\pm0.026\,L_{\odot}$, $L_{\rm A2}=0.288\pm0.030\,L_{\odot}$, and orbital separation $a_{\rm A}=2.078\pm0.010\,R_{\odot}$, based on simultaneous solutions of light and radial velocity curves. The detached binary subsystem XY Leo B is confirmed to be on a wide $\sim 20$-year orbit around the contact system. A second $\sim 23$-year modulation is also detected, which may arise from either stellar magnetic activity or an additional unseen companion. After removing both trends, a coherent residual modulation with a characteristic timescale of $14.2 \pm 0.8$~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 non-conservative evolution, finding strong agreement between the tracks and observed parameters, highlighting the system's value for testing multiple-star evolution.

Figures (6)

• Figure 1: Radial velocity observations and derived models of the contact binary system XY Leo A. The filled circular dots represent the primary component and the open circles the radial secondary. The solid line represents the primary star and the purple dashed line the secondary star. The re-analysed observations are taken from the studies published earlier Hrivnak1984ApJ...285..683HBarden1987ApJ...317..333BPribulla2007AJ....133.1977P (H84, B87, P07). See text for details.
• Figure 2: Superposition of synthetic models (continuum, lines) derived from the simultaneous solution of the XY Leo A contact multiple system with multicolour ($UBVR$) newly obtained TUG T60, TESS Sector 45-46 and photometric observations available in $BVR$ filters published earlier Yakut2003AA...401.1095YDjuravsevic2006PASA...23..154D.
• Figure 3: Period change analysis of the XY Leo A system. The panels display successive stages of the $O\!-\!C$ analysis. The top panel shows the $O\!-\!C$ diagram fitted with two LITE terms and a long-term modulation. The first LITE signal ($P \approx 20$ yr) is attributed to the well-established third body, XY Leo B. The second LITE term (second panel, $P \approx 23$ yr) is included for completeness; however, magnetic activity is considered a more plausible explanation than an additional stellar component. The third panel presents the final residuals, $(O\!-\!C)_{\mathrm{III}}$, which exhibit a coherent quasi-cyclic modulation. The bottom panel shows the Lomb--Scargle periodogram of the $(O\!-\!C)_{\mathrm{III}}$ residuals, revealing a dominant cycle at $P_{(O-C)_{\mathrm{III}}}=14.2 \pm 0.8$ yr, consistent with a magnetic activity cycle of Applegate type.
• Figure 4: Total mass estimates as a function of orbital inclination for the two LITE components. The first corresponds to the confirmed third body (XY Leo B), while the second signal (LITE2) is treated as speculative and may instead arise from magnetic activity. The curves assume Keplerian motion and are shown for illustrative purposes only; the reality of the second modulation remains uncertain due to potential dynamical instability.
• Figure 5: H--R and Mass–Radius diagrams of the four confirmed component stars A1 (black filled circle), A2 (black circle), B1 (orange filled circle), and B2 (orange circle) of the XY Leo multiple system. Observed quantities for other well-studied W-type (blue), A-type (green) contact binaries, and detached systems (purple) are also shown for comparison, along with the zero-age main sequence (red lines) Yakut2005ApJ...629.1055YPols1995MNRAS.274..964P. Filled circles indicate primary components; open circles indicate secondary components.