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S Equulei: A Low Mass Ratio Algol-Type Eclipsing Binary at the Active Phase of Mass Transfer

Orbital period change of a low mass ratio (q = 0.13) Algol-type eclipsing binary, S Equulei, is presented based on the analysis of its century-long times of light minimum. Alternate period changes are discovered to superimpose on a rather rapid period increase with rate of dP/dt = +1.27 × 10-6 days yr-1. The alternate period changes can be explained either by the variation in the gravitational quadruple momentum via the cyclic magnetic activity of the G-type secondary or by the light-time effect via the presence of a third body. Since the third-body assumption is in good agreement with Zola's photometric solution, S Equ may be a truly triple system. The third body is rotating in an eccentric orbit (e′ = 0.37) with an orbital inclination of i′ = 16°, which is rather smaller than that of the eclipsing binary system (i = 88.°3). This indicates that the third body may be captured by the eclipsing pair. New accurate photometric and spectroscopic data over the next decade are very important to verify this conclusion. The rapid period increase suggests that S Equ is a low mass ratio Algol-type binary system in an active phase of mass exchange. To satisfy such a period increase, a conservative mass transfer from the less massive to the more massive components would be of the order dm/dt = 4.94 × 10-8 M⊙ yr-1.