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

Abstract

Aims. The source AC Cnc is a nova-like cataclysmic variable containing a white-dwarf primary with a mass of 0.76 M⊙ and a K2-type secondary with a mass of 0.77 M⊙. 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 16.2yr was found to be superimposed on a long-term period decrease at a rate of P = -1.24(±0.44) × 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 (M3 > 0.097 M⊙) 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 ⊙), the mass-loss rate should be Ṁ2 = -1.65 × 10-10 M⊙/year. By using the enhanced mass loss proposed by Tout & Eggleton (1988), the mass-loss rate should be Ṁ2 = -1.18 × 10-9 M⊙/year. In this case, the Alfén radius is determined to be RA = 5.2R ⊙. 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. © ESO 2007.