2008A&A...480..481B

We report 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 them with published measurements to construct an observed-minus-calculated diagram covering 29 years of observations. The data present a 36 yr period modulation of semi-amplitude ∼40s, with a statistical significance greater than 99.9 percent with respect to a constant period. 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 (Δ P/P) versus the angular velocity of the active star (Ω) 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 ΔP/P∝Ω^–0.7±0.1. Cataclysmic variables below the period gap (with fully convective secondaries) deviate from this relationship by more than 3-σ, with average fractional period changes ≃6 times smaller than those of the systems above the gap.