Flare from the secondary star of an extreme period binary.

This article focuses on the photometric study of BX Trianguli (BX Tri), a short-period, highly active M dwarf semidetached binary system, using ground-based observations and data from the Transiting Exoplanet Survey Satellite (TESS) collected between 2016 and 2023. Analysis via the Wilson–Devinney method reveals that the primary star contributes at least 80% of the system’s luminosity, and spot longitude variability indicates a magnetic activity cycle of approximately 9.4 years. Three flares detected by TESS, including a long-lasting flare near the secondary minimum phase, are inferred to originate from the polar region of the secondary star, with evidence suggesting the flare’s decay is due to an occlusion effect by the primary star. The study also discusses the possibility that ongoing flare activity and orbital dynamics may lead BX Tri to merge into a single star through a process known as mergebursts, highlighting the system’s significance for understanding binary evolution and stellar mergers.