Study of the contact binary AK Herculis: Light-curve analysis and orbital period investigation

AK Her is a very interesting contact binary exhibiting variable light curves and an obvious O'Connell effect, with max I greater than max II. In the present work an extensive study of this system is made covering not only its light curve analysis, but also its orbital period changes in an attempt to give a definite answer regarding the apsidal motion of AK Her. Photoelectric observations of AK Her, obtained between 1985-1987 at the National Observatory of Athens Greece, are presented, analysed and discussed. The light curve analysis was made with the W-D code and the geometric and photometric elements of the system were derived. The O'Connell effect was faced with a two dark spot model. Moreover, the orbital period of the system was examined and two periodicities were detected. Since the first one corresponds to the time interval for which observational data exist and the second was found to be half of the first, it turns out that none of them is true. Two other significant results that came from the present analysis are that the orbital period of AK Her does not follow a sinusoidal variation and that, from the times of minimum light at primary and secondary eclipses, it seems that there is no apsidal motion.