Dwarf novae are erupting cataclysmic variable stars in which a Roche-lobe filling late-type main sequence star(Secondary star) transfers matter to an accretion disc around a white dwarf(Primary star). The gas stream from the secondary feeds material into the disc at a point near its outer rim where a bright spot is formed. The existence of accretion disc makes it have complex physical changes and observed features, including the outburst of the frequent phenomenon and the transitions from a high state to a low state, etc. It will greatly promote the understanding to the theory of the accretion disc and the mass transfer to study these phenomenon. The eclipse provides an excellent opportunity to determine the relative brightness of each light source, geometric position and physical structure. And the times of mid-eclipse also reveals that the orbital period is changing. Therefore, the study of eclipsing dwarf novae is important and meaningful. In the present paper, the photometric observations of a typical eclipsing dwarf novae OY Car are observed and analyzed, and the parameters of the dwarf novae were also studied statistically. Some results are obtained as follows: 1、The orbital period variations of OY Car are analyzed. It is shown that the orbital period of OY Car is decreasing . Since the system consist of a less massive donor star and a more massive gainer star, we would expect the orbital period to increase rather than decrease. Gravitational radiation loss to explain the observed change. The Rappaport et al.(1983) prescription to calculate the orbital period change caused by magnetic braking are found to be able to explain the observed period decrease. However, according to the CV standard model, a donor star should not have magnetic braking. It is therefore possible that the observed downward parabolic change is simply a part of a long-period cyclic variation. On the other hand, recently, a new view that fully convective donors below the period gap are capable of generating significant magnetic fields and can produce MB strong enough。This idea is supported by some observational evidence, and our calculation is consistent with this possibility. These evidences pose a severe challenge to the CV standard model. In addition, a cyclic change with a small amplitude of 27.8s and a period of 14.0 yr is detected. We attempted to interpret the a cyclic change with the light travel-time effect due to an invisible third star in the system. Based on the analysis of the light travel-time effect, we find that the third star may be a critical substellar object between brown dwarf and giant planet. 2、The eclipsing light curves of OY Car are studied, the results reveal that OY Car has the transitions from a high state to a low state in quiescence, which is similar to the magnetic cataclysmic variable and nova-likes. We observed that the magnitude change amplitude between the highest state and the lowest state is about 1mag. From the light curve, we obtained the following information: hot spot is larger than the previous, and brighter(more faint) than before、the expansion of Accretion disk、flicking and the profile of boundary layer. The main reason for the high state is the sudden increase of matter transfer rate and the accretion discs is unstable, and the lower state is mainly caused by mass transfer rate suddenly drops. Starspots theory can well explain the variation of mass transfer, a lower mass transfer rate may be reflect starspots growing near L1 or form elsewhere and drift across it, a higher mass transfer is caused by flare. So far, only a few dwarf novae have been observed to have behavior resembling high or low states. 3、The outburst of the frequent phenomenon have been observed from 28 November 2008 to 14 January 2009, which should be a normal outburst. And the light curves on 23 November 2009 and 24 November 2009 are in the declined stages of a superoutburst. Comparing with the AAVSO's long-term observation result
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