A third body as the origin of the orbital-period variations in the eclipsing binaries TW cas and BE Vul

Abstract

The orbital-period variations in the Algol-type eclipsing binaries TW Cas and BE Vul are analyzed. Cyclic variations of the period have been found, rather than a gradual decrease in the period, as was believed earlier. The cyclic variations of the period of the BE Vul system are superposed on a secular increase in the period that can be explained with a uniform flow of matter from the lower-mass to the higher-mass component, with the total angular momentum being conserved. The mass-transfer rate is about 0.84 ⋗ 10−7M⊙/year. No secular period change is observed for TW Cas. The cyclic variatons in the orbital periods of TW Cas and BE Vul can be explained if the eclipsing binaries move along long-period orbits with high eccentricities (e = 0.74 and 0.6, respectively). In the TW Cas system, the eclipsing binary moves around a third body with a mass M3 > 0.64 M⊙with a period of 203 years; the eclipsing binary in BE Vul moves around a third body with a massM3 > 0.6M⊙with a period of 156 year.