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We formulate an improved time series analysis method for the analysis ofphotometry of active stars. This new Continuous Period Search (CPS) method isapplied to 12 years of V band photometry of the young solar analogue HD 116956(NQ UMa). The new method is developed from the previous Three Stage PeriodAnalysis (TSPA) method. Our improvements are the use of a sliding window inchoosing the modelled datasets, a criterion applied to select the best modelfor each dataset and the computation of the time scale of change of the lightcurve. We test the performance of CPS with simulated and real data. The CPS hasa much improved time resolution which allows us to better investigate fastevolution of stellar light curves. We can also separate between the cases whenthe data is best described by periodic and aperiodic (e.g. constant brightness)models. We find, however, that the performance of the CPS has certainlimitations. It does not determine the correct model complexity in all cases,especially when the underlying light curve is constant and the number ofobservations too small. Also the sensitivity in detecting two close light curveminima is limited and it has a certain amount of intrinsic instability in itsperiod estimation. Using the CPS, we find persistent active longitudes in thestar HD 116956 and a "flip-flop" event that occurred during the year 1999.Assuming that the surface differential rotation of the star causes observableperiod variations in the stellar light curve, we determine the differentialrotation coefficient to be |k|>0.11. The mean timescale of change of the lightcurve during the whole 12 year observing period was T_C=44.1 d, which is of thesame order as the predicted convective turnover time of the star. We alsoinvestigate the presence of activity cycles on the star, but do not find anyconclusive evidence supporting them.

The continuous period search method and its application to the young solar analogue HD 116956