Period changes of AR Lacertae between 1900 and 1989.

The epochs of the primary and secondary minima of AR Lac between 1900 and 1989 are analysed with two nonparametric methods of searching for periodicity in a weighted time point series. An analytical model for transforming the O-C data of eclipsing binaries into the period domain is presented and is applied to AR Lac. The abrupt period changes of AR Lac reported in several previous studies are most probably a consequence of an oversimplified interpretation of the O-C data, since these changes are as likely to be continuous, and even the possibility of a 36yr cycle in the period cannot be definitely ruled out.