δ Scuti light variations as a test for choosing the mechanism of cyclic orbital period changes in Algols

Abstract

The temporal behaviour of the orbital period and pulsation period of the δ Scuti type primary component is studied in the eclipsing binary AB Cas, where the secondary fills its Roche lobe. Orbital period changes observed over 80 years are a superposition of secular variations due to mass exchange between the components and cyclic variations with a quasi-period of about 46 years. At the same time, the period of δ Scuti light variations is found to remain constant to a high accuracy. This result rules out the explanation of cyclic orbital period changes in AB Cas by the presence of a third body and forces us to seek other interpretations. In view of our results, the only mechanism that currently seems to be acceptable for the observed cyclic orbital period changes in AB Cas and, possibly, many other Algols is the mechanism proposed by Applegate in 1992, which explains these modulations by the variations in the quadrupole moment of the component with a convective shell caused by its cyclic magnetic activity.