The probable quadruple systems FK Aql and FZ Del

Abstract

Orbital-period variations of the eclipsing binaries FK Aql and FZ Del are analyzed. For each of the systems, a superposition of two cyclic variations of their orbital periods is found. FK Aql may be a quadruple system that contains two more bodies, besides the eclipsing binary, with masses M₃ ⩾ 1.75M_⊙ and M₄ ⩾ 1.47M_⊙, and the corresponding periods 15 and 82 yrs. This could also be a triple system with a third body of mass M₃ ⩾ 1.75M_⊙ and a period of the long-period orbit P₃ = 15 yrs, or with a third body of mass M₃ ⩾ 1.30M_⊙ and a period of the long-period orbit P₃ = 82 yrs. FZ Del may be a quadruple system with the additional componentmasses M₃ ⩾ 0.2M_⊙ and M₄ ⩾ 0.3M_⊙, with the periods 10.2 and 53.7 yrs. This could also be a triple system with a third-body mass M₃ ⩾ 0.2M_⊙ and a period of the long-period orbit P₃ = 10.2 yrs. In both systems, the residual period variations could be due to magnetic cycles of the secondary. The period variations of the eclipsing binary FZ Del could also be due to apsidal motion, together with the influence of a third body or the effects of magnetic activity.