On The Orbital Period Changes Of Close Binary Stars AH Vir And TV Cas

There are a huge number of stars in the vastness of the universe. Most of them are the close binary systems and some are the triple. Close binaries are often exhibiting a strong Hα and Call H&K emission lines, chromospheric activity, X-ray emission, solar-like activities, mass loss due to star wind, flares, magnetic activity, and orbital period variations, and they are very popular concerned by the astronomers. According to whether their critical Roche lobes are being filled or not, the close binaries systems often are divided into three categories:the detached binary, the semi-contact binary and the contact binary.In this thesis, first of all, we have introduced the basic ways of the study on the orbital period change. Then we discussed the mechanisms of causing the orbital period variations. Finally, we have analyzed the orbital periods of two eclipsing binary systems AH Vir and TV Cas and the main results are as following:1. For AH Vir, we have collected500eclipse times of mimima from literature. With the linear method to fit these data, we obtained the new epoch formula and computed the O-C values. We used the least squares to fit the O-C curve and from the best approximation we found that the orbital period of AH Vir shows a long-term increase with a rate of dP/dt=(2.1869±0.0161)×10-7dyr-1and two small periodic variations with periods of24.79years and44.63years. The periodic variation of the orbital period may be due to the invisible third companion, or the magnetic activity. The long-term period increase may be caused by the less massive component exchanging mass to the more massive one, and we estimated the mass exchange rate to be dM1/dt=1.05×107M(?)yr-12. The orbital period of the close eclipsing binary TV Cas has been analyzed. Through the analysis, we conclude that the orbital period of TV Cas shows a secular decreasing with the rate of dP/dt=-(4.6357±0.1136)×10-7dyr-1. We find that A good fit to the residuals of the O-C by three linear segments rather than a smooth sinusoidal one, meaning that there are two abrupt jumps of the orbital period occurred around1969and2004. This result is contrary to almost all of the conclusions given by the former authors. Since TV Cas is a typical semi-detached binary, the long-term period decrease may not be caused by the secular free mass transfer between the two components, but the loss of mass and angular momentum of the system.