2007AJ....134.1475Q

QX Andromedae is a short-period eclipsing binary in the intermediate-age open cluster NGC 752. Charge-coupled device photometric observations of the close binary system obtained from 2004 November 13 to 2006 November 18 are presented. It is confirmed that the light curves show partial eclipses, and night-to-night intrinsic variations are seen. As in the case of AH Cancri in the old open cluster M67, the light curve of QX And seems to change between A and W types. Both the short- and long-term light variations suggest that QX And shows strong magnetic activity, which is in agreement with its X-ray observations. The symmetric light curves in B and V bands obtained the night of 2004 November 13 were analyzed with the new version of the W-D code. It is found that QX And is a deep overcontact binary system with a high degree of overcontact of f=55.9% and a low mass ratio of q=0.2327, suggesting that it is in the late stage of overcontact evolution. Based on our 23 times of light minimum, including four recently published eclipse times, the orbital period of the eclipsing binary was revised. It was discovered that the orbital period shows a continuous period increase at a rate of dP/dt=+2.48x10^–7 days/yr, which can be interpreted as a mass transfer from the less massive component to the more massive one. As the period increases, the mass ratio of the system will decrease. It may finally evolve into a rapid-rotating single star when it meets the more familiar criterion that the orbital angular momentum is less than 3 times the total spin angular momentum. The existence of QX And in the late evolutionary stage of an overcontact binary in the intermediate-age open cluster NGC 752 indicates that it may undergo strong cluster stellar interaction. This means it had a very short initial orbital period and could have evolved into the present evolutionary state within the cluster age. By comparing with the evolutionary state of TX Cnc in M44, it is estimated that the lifetime of overcontact binaries may be no less than 1 Gyr.