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. 1. The periods of all well-observed W UMa stars are variable.

2. 2. There are no “pure” parabolae in well-observed O - C curves.

3. 3. The periods change in a rather short time and then remain constant for 5,000 - 25,000 E (fig. 2).

4. 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. 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.