EDP Sciences
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Issue A&A
Volume 480, Number 2, March III 2008
Page(s) 481 - 487
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20078596



A&A 480, 481-487 (2008)
DOI: 10.1051/0004-6361:20078596

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

B. W. Borges1, R. Baptista1, C. Papadimitriou2, and O. Giannakis2

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 $(\Delta
P/P)$ 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 $\Delta P/P \propto \Omega^{-0.7
\pm 0.1}$. Cataclysmic variables below the period gap (with fully convective secondaries) deviate from this relationship by more than 3-$\sigma$, with average fractional period changes $\simeq $6 times smaller than those of the systems above the gap.


Key words: accretion, accretion disks -- stars: dwarf novae -- stars: evolution -- stars: binaries: eclipsing -- stars: individual: HT Cas



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Editor-in-Chief: T. Forveille
Letters Editor-in-Chief: J. Alves
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ISSN: 0004-6361 ; e-ISSN: 1432-0746
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Published by: EDP Sciences

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