We report the identification of cyclical changes in the orbital period of the eclipsing dwarf nova Z Cha .
We used times of mid-eclipse collected from the literature and our new eclipse timings to construct an observed-minus-calculated diagram covering 30 years of observations ( 1972-2002 ) .
The data present cyclical variations that can be fitted by a linear plus sinusoidal function with period 28 \pm 2 yr and amplitude 1.0 \pm 0.2 minute .
The statistical significance of this period by an F-test is larger than 99.9 % .
The derived fractional period change , \Delta P / P = 4.4 \times 10 ^ { -7 } , is comparable to that of other short-period cataclysmic variables ( CVs ) , but is one order of magnitude smaller than those of the long-period CVs .
Separate fits to the first and second half of the data lead to ephemerides with quite different cycle periods and amplitudes , indicating that the variation is not sinusoidal or , most probably , is not strictly periodic .
The observed cyclical period change is possibly caused by a solar-type magnetic activity cycle in the secondary star .
An incremental variation in the Roche lobe of the secondary star of \Delta R _ { L 2 } / R _ { L 2 } \simeq 1.7 \times 10 ^ { -4 } is required in order to explain both the observed period change and the modulation of the quiescent brightness previously reported by Ak , Ozkan & Mattei .