EDP Sciences
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Volume 466, Number 2, May I 2007
Page(s) 589 - 594
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20065970

A&A 466, 589-594 (2007)
DOI: 10.1051/0004-6361:20065970

Orbital period changes of the nova-like cataclysmic variable AC Cancri: evidence of magnetic braking and an unseen companion

S.-B. Qian1, 2, 3, Z.-B. Dai1, 2, 3, J.-J. He1, 2, 3, J. Z. Yuan1, 2, 3, F. Y. Xiang4, and M. Zejda5

1  National Astronomical Observatories/Yunnan Observatory, Chinese Academy of Sciences, PO Box 110, 650011 Kunming, PR China
    e-mail: qsb@ynao.ac.cn
2  United Laboratory of Optical Astronomy, Chinese Academy of Science (ULOAC), 100012 Beijing, PR China
3  Graduate School of the CAS, Beijing, PR China
4  Physics Department, Xiangtan University, 411105 Xiangtan, Funan Province, PR China
    e-mail: fyxiang@xtu.edu.cn
5  Institute of Theoretical Physics and Astrophysics, Masaryk University, 611 37 Brno, Czech Republic
    e-mail: zejda@physics.muni.cz

(Received 5 July 2006 / Accepted 27 October 2006)

Aims.The source AC Cnc is a nova-like cataclysmic variable containing a white-dwarf primary with a mass of $0.76\,M_{\odot}$ and a K2-type secondary with a mass of $0.77\,M_{\odot}$. We intend to study its period changes and search for evidence of magnetic braking and unseen third body.
Methods.The period changes were investigated based on the analysis of the O-C curve, which is formed by one new eclipse time together with the others compiled from the literature.
Results.A cyclic change with a period of 16.2 yr was found to be superimposed on a long-term period decrease at a rate of $\dot{P}=-1.24(\pm0.44)\times{10^{-8}}$ days/year.
Conclusions.It is shown that the mechanism of magnetic activity-driven changes in the quadrupole momentum of the secondary star (Applegate's mechanism) does not explain it easily. This period oscillation was plausibly interpreted by a light-travel time effect caused by the presence of a cool M-type dwarf companion ($M_3>0.097\,M_{\odot}$) in a long orbit (16.2 yr) around the binary. Since the masses of both components are nearly the same, the mass transfer from the lobe-filling secondary to the primary is not efficient to cause the continuous period decrease. It may be strong evidence of an enhanced magnetic stellar wind from the K2-type component. If the Alfén radius of the cool secondary is the same as that of the Sun (i.e., RA = 15 $R_{\odot}$), the mass-loss rate should be $\dot{M_2}=-1.65\times{10^{-10}}~M_{\odot}$/year. By using the enhanced mass loss proposed by Tout & Eggleton (1988), the mass-loss rate should be $\dot{M_2}=-1.18\times{10^{-9}}~M_{\odot}$/year. In this case, the Alfén radius is determined to be RA = 5.2 $R_{\odot}$. However, the long-term decrease of the period may be only a part of a long-period (>100 yr) oscillation caused by the presence of an additional body. To check the conclusions, new precise times of light minimum will be required.

Key words: stars: novae, cataclysmic variables -- stars: binaries: eclipsing -- stars: individual: AC Cnc -- stars: evolution -- stars: mass-loss -- stars: binaries: close

© ESO 2007