Probing Period Variability Mechanisms in SW Cygni through Eclipse Timing Variations

Abstract This paper presents a comprehensive study of the Algol-type eclipsing binary SW Cyg using new high-precision TESS and AAVSO photometric data together with historical observations. In total, 442 eclipse timings were compiled for analysis. Through detailed light-curve modeling with the Wilson–Devinney code, combined with the adopted spectral-type-based mass calibration for the primary component, the absolute physical parameters of SW Cyg were derived. We investigate the complex O − C curve through a systematic multimodel analysis involving six physical mechanisms, optimized via global search algorithms, local optimization, and Bayesian Markov Chain Monte Carlo sampling. A rapid increase in the secular period of approximately +2.6 × 10 −6 days yr −1 is confirmed, consistent with the mass transfer from the Roche-lobe-filling secondary to the primary. Statistical comparisons, together with assessments of the physical feasibility of the Applegate mechanism, luminosity and astrometric constraints, and simple model priority, identify Model (e), i.e., secular change superimposes two different kinds of cyclic variation terms, as the most robust explanation. Our findings demonstrate that the observed period variations are driven by a combination of secular evolution, the secondary’s 82 yr magnetic activity cycle, and a light-travel time effect induced by a circumbinary companion with a minimum mass of 0.8 M ⊙ in a 36 yr orbit.