1999A&A...351..212J

Standard Johnson UBVRI photometry of V1794 Cyg (HD 199178) between 1975 and 1995 is analysed. Instead of the traditional constant period ephemeris, we determine the seasonal periodicities (P_phot) and the primary and secondary minima epochs (t_min,1, t_min,2) of the normalized UBVRI magnitudes using 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 P_phot are utilized in modelling the mean (M) and total amplitude (A) of contemporary light curves in UBVRI. TSPA reveals that the long-term M and A changes of V1794 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 3.3175d period detected with the 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 distributed. The activity centres in V1794 Cyg are rarely disrupted, and most probably undergo continuous longitudinal migration, because only one abrupt disruption is observed during 20 years. As for differential rotation, the irregular 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 ``butterfly'' diagram.