We report the identification of cyclical changes in the orbital period of the eclipsing dwarf novae V2051 Ophiuchi and V4140 Sagitarii .
We used sets of white dwarf mid-eclipse timings to construct observed-minus-calculated diagrams covering , respectively , 25 and 16 years of observations .
The V2051 Oph data present cyclical variations that can be fitted by a linear plus sinusoidal function with period 22 \pm 2 yr and amplitude 17 \pm 3 s. The statistical significance of this period by an F-test is larger than 99.9 per cent .
The V4140 Sgr data present cyclical variations of similar amplitude and period 6.9 \pm 0.3 yr which are statistically significant at the 99.7 per cent level .
We derive upper limits for secular period changes of | \hbox { \ . { P } } | < 3 \times 10 ^ { -12 } and | \hbox { \ . { P } } | < 1.8 \times 10 ^ { -11 } , respectively for V2051 Oph and V4140 Sgr .

We combined our results with those in the literature to construct a diagram of the amplitude versus period of the modulation for a sample of 11 eclipsing cataclysmic variables ( CVs ) .
If the cyclical period changes are the consequence of a solar-type magnetic activity cycle in the secondary star , then magnetic activity is a widespread phenomenon in CVs , being equally common among long- and short-period systems .
This gives independent evidence that the magnetic field ( and activity ) of the secondary stars of CVs do not disappear when they become fully convective .
We also find that the fractional cycle period changes of the short-period CVs are systematically smaller than those of the long-period CVs .