  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 +/- 2 yr and amplitude 17 +/- 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
+/- 0.3 yr which are statistically significant at the 99.7 per cent level. We
derive upper limits for secular period changes of |dP/dt| < 3x10^{-12} and
|dP/dt| < 1.8x10^{-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.
