The Triple System V1371 Tau: An Eclipsing Binary with an Outer Be Star

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Abstract
Although triple systems are common, their orbital dynamics and stellar evolution remain poorly understood. We investigated the V1371 Tau system using TESS photometry, multiepoch spectroscopy, and recent interferometric data, confirming it as a rare triple system consisting of a eclipsing binary orbited by a classical Be star, with a spectral classification of (B1V + B0V) + B0Ve. The eclipsing binary exhibits an orbital period of ≈34 days, and the Be star orbits the inner pair on a timescale of a few years. Weak H α emission lines suggest the presence of a Keplerian disk with variability on a timescale of months around the Be star, and nearly constant V / R ratio no detectable asymmetry variations. Besides the eclipses, frequencies at 0.24 and 0.26 cycles day –1 dominate the photometric variability. Higher-frequency signals are present, which appear associated with nonradial pulsations. The eclipsing pair ( i ≈ 90 ∘ ) shows projected rotational velocities of 160 and 200 km s −1 . The Be star’s measured v sin i ≈ 250 km s −1 implies a critical rotation fraction between 0.44 and 0.76 for plausible inclinations, significantly faster than the eclipsing components. The shallower eclipses in the KELT data compared to TESS suggest a variation in orbital inclination, possibly induced by Kozai–Lidov cycles from the outer Be star. The evolution analysis suggests that all components are massive main-sequence stars, with the secondary star in the eclipsing binary being overluminous. This study emphasizes the complexity of triple systems with Be stars and provides a basis for future research on their formation, evolution, and dynamics.

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