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A detailed analysis of an extensive collection of primary visual, photographic and photoelectric minima of Beta Lyrae was performed. After removing the effect of the increase in orbital period we applied the Fourier analysis to the 0-C residuals. The most prominent periodicity is 283 days with amplitude about 0.05 days. It confirms the period found in the photometric light curves of Beta Lyrae [Van Hamme, Wilson and Guinan (1995)]. The 283-d quasi-variation of the period may be explained by assymetries of the light curve near the minima caused by some instabilities in disc and/or hot spot(s) brightness.
AN Tau has been classified as the β Lyrae-type variable. The only visual light curve was published 55 years ago. The star was observed photoelectrically during a few last years at Mt. Suhora and Skalnaté Pleso Observatories. The complete light curve was obtained in B and V band passes. The preliminary physical parameters of the system were obtained using the Wilson-Devinney code. From the period study of AN Tau we derived new period of the system being 1d.6143843, which differs by 0d.00026 from the value obtained earlier. The orbital period of AN Tau seems to be constant.
About 6000 times of minima for 120 W Ursae Majoris stars were collected by Mrs. Z. Frasińska and the writer in the Cracow Astronomical Observatory. From this material 15 O - C diagrams (fig. 1) with large number of minima were taken into consideration. Each of them covered more than 30,000 epochs. The analysis of O - C curves leads to the following conclusions:
1. The periods of all well-observed W UMa stars are variable.
2. There are no “pure” parabolae in well-observed O - C curves.
3. The periods change in a rather short time and then remain constant for 5,000 - 25,000 E (fig. 2).
4. The number of increases and decreases of periods is approximately the same. The mean absolute change of period of 15 investigated stars amounts to 0.d00000365 = 0.P0000105. (The mean increase of period = 0.d00000386 = 0.P0000109, the mean decrease of period = 0.d00000349 = 0.P0000102).
5. The mean absolute change of period for 15 investigated stars was plotted versus parameter q (mass ratio = m2/m1) (fig. 3). Systems with almost equal masses of stars have a tendency to show violent changes of period.
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