Abstract We present results of CCD photometric observations of the short-period W UMa-type contact binary system, RZ Com. The light curve of the binary has changed from W-subtype to A-subtype from 1998 to 2003, then back to W-subtype in 2004. An analysis was carried out using the 2003 version of the Wilson-Devinney code. It is confirmed that RZ Com is a low-degree, overcontact f = 20.1%(±7.4%) binary system with a high inclination of i = 8140(±040), and a mass ratio q = 2.351(±0.031). Combining four newly determined times of light minimum with others in the literature, the variations in orbital period is examined. A small-amplitude oscillation (A=0.0065d), with a period of 41.5 year, is discovered superimposed on a long-term increase at rate dP/dt = +3.97×10−8d yr−1. The period oscillation can be explained either by the light-time effect due to the presence of an unseen third body, or by cycles of magnetic activity on the components. Combining our photometric solution with the spectroscopic elements obtained by Mclean & Hilditch, the absolute dimensions of RZ Com are: M1 = 1.14(±0.19)M⊙, M2 = 0.50(±0.09)M⊙, R1 = 1.12(±0.01)R⊙, R2 = 0.78(±0.01)R⊙ and A = 2.41(±0.02)R⊙.
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