The orbital period changes in some near contact eclipsing binaries systems

In this thesis, firstly the main features of eclipsing binaries, Roche Geometry and the Orbital Period Variations in these systems including O-C method were given. The general information for near contact binaries (NCBs), which are the target objects for this study were briefed. And also, the distribution of the absolute parameters of NBCs was examined. The orbital period variations of NCBs and their general character are examined.In this study, the orbital period variations of four NCBs, BX And, ST Aqr, V388 Cyg and V456 Cyg, which are different evolutionary stages, are investigated. It was found that the O-C variation of BX And has a cyclic character with a period of about 68 years. It can be explained with probable third body in this systems. The orbital period variation of ST Aqr is studied for the first time and it was proposed that the orbital period of systems is decreasing with a rate of 2,6 sec/century. The magnetic activity of cooler compenent of system can be responsible for the decreasing orbital period of ST Aqr. The O-C diagram of V388 Cyg indicates downward parabola. This means that orbital period of system is also decreasing with a rate of 3,4 sec/ century. The mass transfer from the primary componenet to secondary one is suggested for the reason of the decreasing orbital period of V388 Cyg. The O-C analyze of V456 Cyg indicates that its orbital period is increasing with a rate of 0, 9 sec/ century.