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

Abstract

AK Her is a very interesting contact binary exhibiting variable lightcurves and an obvious O'Connell effect, with max I greater than max II. Inthe present work an extensive study of this system is made covering notonly its light curve analysis, but also its orbital period changes in anattempt to give a definite answer regarding the apsidal motion of AK Her.Photoelectric observations of AK Her, obtained between 1985–1987 at theNational Observatory of Athens Greece, are presented, analysed anddiscussed. The light curve analysis was made with the W-D code and thegeometric and photometric elements of the system were derived. TheO'Connell effect was faced with a two dark spot model. Moreover, theorbital period of the system was examined and two periodicities weredetected. Since the first one corresponds to the time interval for whichobservational 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 thatcame from the present analysis are that the orbital period of AK Her doesnot follow a sinusoidal variation and that, from the times of minimumlight at primary and secondary eclipses, it seems that there is no apsidalmotion.