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Solar spots appear to decay linearly proportional to their size. The decayrate of solar spots is directly related to magnetic diffusivity, which itselfis a key quantity for the length of a magnetic-activity cycle. Is a linear spotdecay also seen on other stars, and is this in agreement with the large rangeof solar and stellar activity cycle lengths? We investigate the evolution ofstarspots on the rapidly-rotating ($P_{\rm rot}$ $\approx$ 24 d) K0 giant XXTri, using consecutive time-series Doppler images. Our aim is to obtain awell-sampled movie of the stellar surface over many years, and thereby detectand quantify a starspot decay law for further comparison with the Sun. Weobtained continuous high-resolution and phase-resolved spectroscopy with the1.2-m robotic STELLA telescope on Tenerife over six years. For each observingseason, we obtained between 5 to 7 independent Doppler images, one per stellarrotation, making up a total of 36 maps. To quantify starspot area decay andgrowth, we match the observed images with simplified spot models based on aMonte Carlo approach. It is shown that the surface of XX Tri is covered withlarge high-latitude and even polar spots and with occasional small equatorialspots. Just over the course of six years, we see a systematically changing spotdistribution with various timescales and morphology, such as spot fragmentationand spot merging as well as spot decay and formation. An average linear decayof $D$ = $-$0.022 $\pm$ 0.002 SH/day is inferred. We found evidence of anactive longitude in phase toward the (unseen) companion star. Furthermore, wedetect a weak solar-like differential rotation with a surface shear of $\alpha$= 0.016 $\pm$ 0.003. From the decay rate, we determine a turbulent diffusivityof $\eta_T$ = (6.3 $\pm$ 0.5) $\times$ 10$^{14}$ cm$^2$/s and predict amagnetic activity cycle of $\approx$ 26 $\pm$ 6 years.

Spot evolution on the red giant star XX Triangulum. A starspot-decay analysis based on time-series Doppler imaging