Studying orbital period variations of XY Leo through updated eclipse times and multi-model analysis

Abstract

This study revisits the orbital period variation of the short-period eclipsing binary system XY Leo, a W-subtype W Ursae Majoris (W UMa) contact binary, by analyzing 30 newly collected mid-eclipse times from three telescopes between 2015 and 2021. The obtained light curves and residuals showed no deviations attributable to effects like pulsations or starspots. The updated $O-C$ diagram, covering 77 years and extended by four years of new data, revealed minor deviations from previously published models. Three models were applied using MCMC sampling to analyze the orbital period variations of XY Leo. Model I includes the linear ephemeris, a quadratic term, and the Light Travel Time (LTT) effect of a 3rd body orbiting the central binary system. Models II and III add a sinusoidal function for the magnetic cycle and the LTT effect of a 4th body to Model I, respectively. Updated orbital parameters for the 3rd body are slightly larger than earlier estimates, except for $e_3$. Applegate mechanism analysis of the magnetic cycle indicated possible magnetic activity, though the derived period (28.09 years) exceeds the expected range. For the first time, Model III considers a 4th body, yielding the lowest RMS and systematic error, suggesting better compatibility with the $O-C$ diagram. Orbital stability tests confirmed stable orbits for Models I and II over one Myr, while Model III showed no stability. Persistent oscillations in Model I residuals suggest that the orbital period variation requires additional explanations, such as magnetic cycles or the influence of further companions.