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

¹ Departamento de Física, Universidade Federal de Santa Catarina, CEP 88040-900, Florianópolis, Brazil e-mail: [bernardo;bap]@astro.ufsc.br
² Institute of Astronomy and Astrophysics, National Observatory of Athens, PO Box 20048, Athens 11810, Greece e-mail: [cpap;og]@astro.noa.gr

Received: 31 August 2007
Accepted: 26 December 2007

Abstract

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 versus the angular velocity of the active star  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 . 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.