A new CCD photometric investigation of the short-period close binary AP Leonis

New CCD photometric light curves in the B, V, and R bands of the short-period close binary AP Leonis are presented. A photometric analysis with our symmetric light curves suggests that AP Leo is an overcontact binary with a degree of overcontact of 24.9%. Since the O - C values of photographic and visual times of light minimum showed a large scatter (up to 0.06 days), all of the period changes proposed for the eclipsing binary by previous investigators are not reliable. In this paper the orbital period changes of AP Leo are analyzed based on all published CCD and photoelectric eclipse times. A small-amplitude cyclic oscillation, with a period of 22.4 yr and an amplitude of 0.0049 days, is discovered to be superposed on a secular period decrease at a rate of dP/dt = - 1:08; 10(-7) days yr(-1). The continuous period decrease may be caused by angular momentum loss or a combination of the mass transfer from the primary to the secondary and angular momentum loss. The cyclic period change may indicate that AP Leo is a triple system containing a cool dwarf third component. If this is true, it is possible that this third component plays an important role in the origin and evolution of the overcontact system by removing angular momentum from the central system, and that it makes the eclipsing pair have a low angular momentum and a short initial orbital period ( e. g., P < 5 days). In that case, the initially detached system evolves into an overcontact configuration via magnetic torques from stellar winds. On the other hand, the rapid rotation of the solar-type components (spectral type G0) and the variations of the light curve indicate a high degree of magnetic activity from the spin-up of the components. Both the long-term period decrease and the oscillation can plausibly be interpreted by magnetic activity (i.e., enhanced magnetic stellar wind and activity cycles).