  We report the identification of cyclical changes in the orbital period of the
eclipsing cataclysmic variable HT Cas. We measured new white dwarf mid-eclipse
timings and combined with published measurements to construct an
observed-minus-calculated diagram covering 29 years of observations. The data
present cyclical variations that can be fitted by a linear plus sinusoidal
function with period 36 yr and semi-amplitude ~ 40 s. The statistical
significance of this period by an F-test is larger than 99.9 per cent. We
combine our results with those in the literature to revisit the issue of
cyclical period changes in cataclysmic variables and their interpretation in
terms of a solar-type magnetic activity cycle in the secondary star. A diagram
of fractional period change (Delta P/P) versus the angular velocity of the
active star (Omega) for cataclysmic variables, RS CVn, W UMa and Algols reveal
that close binaries with periods above the gap (secondaries with convective
envelopes) satisfy a relationship Delta P/P \propto Omega^{-0.7+/-0.1}.
Cataclysmic variables below the period gap (with fully convective secondaries)
deviate from this relationship by more than 3-sigma, with average fractional
period changes ~ 5 times smaller than those of the systems above the gap.
