On The Orbital Period Changes Of RS CVn-type Binaries SZ Psc And ER Vul

The RS CVn-type group are the binary systems showing chromospheric and magnetic activity. These eclipsing binary systems possess a strong Hαand CaII H &K emission lines, and their orbital periods are usually changed. In order to convenient for research, the RS CVn-type binary are divided into three categories: (1) the short-period RS CVn-type binaries whose orbital period are less than 1 day; (2) the normal RS CVn-type binaries whose orbital period are from 1 to 14 days; and (3) the long-period RS CVn-type binaries whose orbital period are longer than 14 days. In 1955, Kopal divided the binaries into three types: detached binaries, semi-detached binaries and contact binaries based on the conditions of if the Roche lobe of two components is filled. For the RS CVn-type binaries, their Roche lobes of two components are not filled. They are the typical detached binaries. The interaction between the components of the detached binaries is week because its Roche lobe of two components is not filled. If one of the components of the binary system is fill its Roche lobe, it is so called semi-detached binary. Algol-type binaries are the typical of the semi-detached binary systems. For the Algol-type binaries, the system is composed of a massive main sequence star and a small mass giant or subgiant star. The massive main sequence star's Roche lobe is not filled and the small mass giant or subgiant's Roche lobe is filled. The contact binary is the system whose Roche lobe of two components is filled. These systems have strong interactions.In the present thesis, firstly, we have introduced the basic methods of analyzing the orbital period and given the mechanisms of the orbital period variation. Then, we have studied the orbital period changes of two RS CVn-type binaries of SZ Psc and ER Vul, and given the following results:1. The orbital period of RS CVn-type binaries SZ Psc was analyzed, we found its orbital period is changing. It was found that a sinusoidal variation, with a period of 55.5yr and an amplitude of 0.2667 d, is superimposed on a long-term period decrease at a rate of dP / dt = -2 .7×10^-8days /day. The long-term period decrease at a rate of dP / dt = -2.7×10 ^-8days /day may be caused by enhanced magnetic wind braking, applying dP / dt =-2.7×10^-8days /day, we obtain R A= 72.1R; Our study shows that the period variation of SZ Psc can be explained by the magnetic activity of the K star.2. It is found that the orbital period shows two cyclic variations with different periods: a long-period variation of 46.2 years, and a short-period variation of 17.2 years, respectively, while it undergoes a possible continuous decrease with a rate of dP / dt = -3.4×10^-8days /year. ER Vul is a typical detached binary, we get the computed value is the same order of magnitude with the observed value, therefore the secular decrease in the orbital period of the system may be caused by magnetic braking of the completely convective component. Two period oscillations may be caused by magnetic activity cycle and light time effect due to the third body.