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 (Pphot) and the primary and secondary minima epochs (tmin,1, tmin,2) of the normalized UBVRI magnitudes using the three stage period analysis (TSPA) and complementary methods. Our tmin,1 and tmin,2 estimates with variable Pphot can adapt easily to both differential rotation and longitudinal activity migration. The seasonal Pphot 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 V 1794 Cyg are unpredictable. We search for active longitudes from the tmin,1 and tmin,2 series of time points with nonparametric methods. The critical level of 0.0029 for the best 3.d3175 period detected with the Kuiper method is high, but exceeds the 0.001 significance for rejecting the hypothesis that the phases of tmin,1 and tmin,2 are randomly distributed. The activity centres in V 1794 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 Pphot are 7.5%. The surprisingly regular 3.3% changes of yearly Pphot may provide a stellar analogy of the solar “butterfly” diagram.