Four decades of photometry of XX Trianguli, ‘the most spotted star’ in the sky

Context. Over the past 40 years the brightness variations of XX Tri, a single-line RS Canum Venaticorum (RS CVn)-type binary system with a synchronised K-giant primary, has exceeded one magnitude in the V band. Although these changes are primarily caused by starspots, an additional activity-related mechanism may also be behind the long-term trend of overall brightness increase. Aims. By compiling the most complete photometric data set so far, we attempt to examine how the nature of seasonal-to-decadal changes can be linked to global magnetism. Methods. To find the long-term activity cycles and their properties a time-frequency analyser code is used. We divided the entire data set into four consecutive intervals, for which we separately determined the Fourier spectra around the orbital period. This is performed with a Fourier-transformation-based frequency analyser tool. Results. The long-term brightening of XX Tri was accompanied by a gradual increase in the effective temperature, which resulted in a blueing shift in the Herzsprung-Russell diagram. In the long term, a constant cycle of about four years is most strongly present in the entire data. We also found a modulation of about 11 years and a slowly decreasing cycle of about 5.7–5.2 years. From the seasonal datasets, we found that the most dominant rotation-related periods are scattered around the orbital period. From this we infer a solar-type surface differential rotation, although the surface shear is significantly smaller than that of the Sun. Conclusions. The four-year cycle indicates flip-flop-like behaviour; during this time, the two-to-three active longitudes usually present on the stellar surface are rearranged. The magnitude range changes in the long term cannot be interpreted solely as changes in the number and size of spots; the unspotted brightness of XX Tri has also increased over the decades. This should alert users of photometric spot models to reconsider the basic concept of constant unspotted brightness in similar cases.