Period changes in W UMa-type eclipsing binaries: DK Cygni, V401 Cygni, AD Phoenicis and YSextantis

We present a period analysis of the four well-known W UMa-type eclipsing binaries DK Cyg (), V401 Cyg (), AD Phe () and Y Sex (). Several new times of minimum light, recorded photoelectrically, have been gathered. Analysis of all available eclipse timings of the DK Cygni and V401 Cygni has confirmed a significant increase in period of and day cycle-1, respectively. A simultaneous solution of the and R light curves was computed for V401 Cyg using the Binary Maker 2.0 synthetic lightcurve software. This solution indicates that V401 Cyg is in contact with a filing factor of 46%. The period of AD Phe seems to be constant. Period changes of Y Sex could be explain by a light-time effect caused by a third body in an eccentric orbit with a period of 58 years.