In this thesis I present the investigation of W UMa-type binaries based on photometric observation and statistic analysis. First of all, an introduction about the observational and statistic properties of W UMa-type binaries are presented. Then, the dynamical stability and evolutionary status of W UMa-type binaries are studied statistically. The photometric analysis of two W UMa-type binaries are described and the photometric resolutions are given in the following part. Besides, the changes in the orbital periods of the two systems are analyzed based on O-C diagrams, and the possible explanations for the changes are discussed. The results obtained in this thesis are mainly consisted as follows: 1. An sample including 130 W UMa-type systems is analyzed statistically based upon their orbital and physical parameters. It is found that there is no evolutionary difference between this two subtypes of W UMa-type systems. A- and W-type systems are probably in different stages of thermal Relaxation Oscillations. The mass-radius and mass-luminosity diagrams indicate that a large amount of energy should be transferred between the two components of W UMa-type systems. From the relation between the relative energy transfer rate of the primary component and the mass ratio of W UMa-type systems, it can be seen that the relative energy transfer rate peaks at a mass ratio of about 0.55. As indicated in the M-L diagram which shows the relation between the total mass and the total angular momentum of W UMa-type systems, there is a tendency for a decreasing total mass with decreasing mass ratio. When their mass ratios are decreased to the cut-off mass ratio (about 0.076), W UMa-type systems will merge into the rapidly rotating stars within 1000-10000 yr, such as blue stragglers and FK Com stars. 2. New CCD photometric observations for BI CVn were carried out with the 1-meter reflecting telescope at Yunnan Observatory. By means of the Wilson-Devinney (WD) code, a simultaneous solution of light and radial velocity curves with dark spot model and the absolute parameters are obtained for this contact binary system. A discussion is made upon the distance to the system and the evolutionary status of BI CVn. 3. According to the O-C diagrams, the changes in the orbital period of BI CVn are studied. The mass transfer rate between the components is estimated based on the secular decrease in the orbital period. There is also a small-amplitude oscillation superposed on the secular decrease in its orbital period change, which might be caused either by the presence of a third body or the solar-like magnetic activity cycles of the components in the system. Then the related parameters are estimated corresponding to this two possible explanations. 4. A preliminary photometric analysis is made and the photometric resolution is given for the BV light curves of the W UMa-type system CW Cas. It confirmed the classification of W-type for this marginal contact system. Based on O-C diagram, the changes in the orbital period of CW Cas are studied preliminarily. It is found that there is a tendency with secular decrease in the orbital period changes of CW Cas.
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