Cyclical period changes in HT Cassiopeiae: a difference between systems above and below the period gap

Aims:We report identification of cyclical changes in the orbital period of the eclipsing cataclysmic variable HT Cas.
Methods: 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.
Results: The data present a 36 yr period modulation of semi-amplitude ~40 s, with a statistical significance greater than 99.9 percent with respect to a constant period.
Conclusions: 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 Δ P/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.

Based on observations

made at the Astronomical Station Kryoneri, owned by the National

Observatory of Athens, Greece.