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

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

Abstract

Aims.The source AC Cnc is a nova-like cataclysmic variable containing a white-dwarf primary with a mass of and a K2-type secondary with a mass of . 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  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 () 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., R_A = 15 ), the mass-loss rate should be /year. By using the enhanced mass loss proposed by Tout & Eggleton (1988), the mass-loss rate should be /year. In this case, the Alfén radius is determined to be R_A = 5.2 . 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.