XX Tri photometry

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-Russ ell diagram. In the long term, a constant cycle of about 4 years is most strongl y present in the entire data. Besides, we also found a modulation of about 11 ye ars, 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. The 4-year cycle indicates flip-flop-like behavior: during this time, the 2-3 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.