SIMBAD references

AO Cam and AM Leo are two short-period overcontact binary systems whose physical properties are nearly the same. In the present paper, three CCD times of minimum light for AO Cam and two photoelectric eclipse times for AM Leo are presented. Combining the new determined eclipse times with others compiled from the literature, the behavior of their O-C variations were investigated. For AO Cam, its orbital period is revised, and it is found that the period shows a cyclic variation with a period of 20.1 yr and an amplitude of 0.0047 days. In AM Leo, a new period increase has occurred recently. Two possibilities of period variations, sudden and continuous changes, are discussed. The period changes of both systems cannot be explained by apsidal motion, since both the primary and the secondary eclipse times vary in the same O-C trend. Of the three mechanisms of magnetic activity cycles, internal mass motions, and the presence of a third body, we think that the last is more plausible. Assuming that the period changes are due to third bodies revolving around the eclipsing pairs, the parameters of the third-component stars are determined. If it exists, the third body in AO Cam is revolving in a circular orbit, whereas that in AM Leo is revolving in an eccentric orbit (e'=0.58). No secular period changes were discovered in either overcontact binary star, which is in agreement with the recent statistical relation of Qian. This may suggest that AO Cam and AM Leo are in transition between the angular momentum loss-controlled and the thermal relaxation oscillation-controlled stages of the evolutionary scheme recently proposed by Qian.