Time series analysis of V 1794 Cygni long-term photometry

Standard Johnson UBVRI photometry of V 1794 Cyg (HD199178) between 1975 and 1995 is analysed. Instead of the traditional constant period ephemeris, we determine the seasonal periodicities (P-phot) and the primary and secondar y minima epochs (t(min,1), t(min,2)) Of the normalized UBVRI magnitudes usi ng the three stage period analysis (TSPA) and complementary methods. Our t( min,1) and t(min,2) estimates with variable P-phot can adapt easily to both differential rotation and longitudinal activity migration. The seasonal Pp hot are utilized in modelling the mean (M) and total amplitude (A) of conte mporary Tight curves in UBVRI. TSPA reveals that the long-term Ai and A cha nges of V 1794 Cyg are unpredictable. We search for active longitudes from the t(min,1) and t(min,2) series of time points with nonparametric methods. The critical level of 0.0029 for the best 3.(d)3175 period detected with t he Kuiper method is high, but exceeds the 0.001 significance for rejecting the hypothesis that the phases of t(min,1) and t(min,2) are randomly distri buted. The activity centres in V 1794 Cyg are rarely disrupted, and most pr obably undergo continuous longitudinal migration, because only one abrupt d isruption is observed during 20 years. As for differential rotation, the ir regular changes of seasonal P-phot are 7.5%. The surprisingly regular 3.3% changes of yearly P-phot may provide a stellar analogy of the solar "butter fly" diagram.