2007A&A...466..589Q

The source AC Cnc is a nova-like cataclysmic variable containing a white-dwarf primary with a mass of 0.76M_☉ and a K2-type secondary with a mass of 0.77M_☉. We intend to study its period changes and search for evidence of magnetic braking and unseen third body. 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. A cyclic change with a period of 16.2yr was found to be superimposed on a long-term period decrease at a rate of {dot}(P)=-1.24(±0.44)x10^–8days/year. 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>0.097M_☉) in a long orbit (16.2yr) 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 Alfen radius of the cool secondary is the same as that of the Sun (i.e., R_A=15R_☉), the mass-loss rate should be {dot}(M₂)=-1.65x10^–10M_☉/year. By using the enhanced mass loss proposed by Tout & Eggleton (1988), the mass-loss rate should be {dot}(M₂)=-1.18x10^–9 M_☉/year. In this case, the Alfen radius is determined to be R_A=5.2R_☉. However, the long-term decrease of the period may be only a part of a long-period (>100yr) oscillation caused by the presence of an additional body. To check the conclusions, new precise times of light minimum will be required.