An Orbital Period Study of the W UMa-Type Binary RZ Comae Berenicis

New photoelectric and CCD photometry observations of a short-period W UMa-type binary system, RZ Com, are presented. The light curves of Broglia (1960, Contr. Milano-Merate, 165) were symmetric in V band, while the present light curve shows a typical O’Connell effect, with Maximum I brighter than Maximum II by 0.015 mag. It is found that the light curve of the binary star has changed from W-subtype to A-subtype according to Binnendijk’s classification. This variation may be caused by the activity of dark spot on the primary component. Combining four newly determined times of the light minimum with others published in the literature, the orbital period change of the system was investigated. A small-amplitude oscillation (A = 0.0058 \,d), with a period of 44.8 yr has been discovered to be superimposed on a long-term period increase with a rate of dP/dt = + 4.12 × 10⁻⁸ \,d \,yr⁻¹. The period oscillation can be explained either by the light-time effect via the presence of an unseen third body or by magnetic-activity cycles of the components. The mass ratio of RZ Com is $q = 0.43$. The secular period variation is in agreement with the conclusions of Qian (2001, MNRAS, 328, 914; 2003, MNRAS, 342, 1260). This indicates that it is on the TRO-controlled stage of the evolutionary scheme proposed by Qian.