2000A&A...356..643O

We studied the long-term light behaviour of ten rapidly-rotating active stars (LQ Hya, V833 Tau, EI Eri, V711 Tau, BY Dra, HU Vir, IL Hya, VY Ari, HK Lac, IM Peg) with Fourier analysis. Using photometric databases of up to 30 years in length, we search for periodic or quasi-periodic long-term variations of the overall light levels. Out of the ten stars studied, nine show cyclic variability and six of those seem to vary on two or multiple time-scales. Using additional data from the literature, we found that cycle lengths were generally longer for stars with longer rotation periods. The inverse Rossby numbers of these stars show a correlation with ω_cyc/Ω(=P_rot/P_cyc) calculated from the shortest cycle length. The cycle lengths of several stars agree well with other types of cycles determined earlier. Using artificial test data, we found that the cyclic variation of the average light level is determined by the change in the spot/plage coverage alone and that spot migration does not play a significant role in the shape or length of the long-term brightness changes.