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

TL;DR

This study identifies cyclical orbital period changes in HT Cas, revealing a significant 36-year cycle likely linked to magnetic activity, and compares period variations between systems above and below the period gap.

Contribution

It provides the first detailed analysis of cyclical period changes in HT Cas and demonstrates a clear difference in magnetic activity-related period variations between systems above and below the period gap.

Findings

Cyclical period variation in HT Cas with a 36-year cycle and ~40 s amplitude.

Systems above the period gap follow a specific period change relationship with stellar rotation.

Systems below the period gap show smaller fractional period changes and deviate from the above-gap trend.

Abstract

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.