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Magnetic spots on low-mass stars can be traced and characterised usingmulti-band photometric light curves. Here we analyse an extensive data set forone active star, V1598Cyg, a known variable K dwarf which is either pre-mainsequence and/or in a close binary system. Our light curve contains 2854photometric data points, mostly in $V$, $R_c$, $I_c$, but also in $U$, $B$ and$H\alpha$, with a total baseline of about 4yr, obtained with small telescopesas part of the HOYS project. We find that V1598Cyg is a very fast rotator witha period of 0.8246 days and varying amplitudes in all filters, best explainedas a signature of strong magnetic activity and spots. We fit the photometricamplitudes in $V$, $R_c$, $I_c$ and use them to estimate spot properties, usinga grid-based method that is also propagating uncertainties. We verify themethod on a partial data set with high cadence and all five broad-band filters.The method yields spot temperatures and fractional spot coverage with typicaluncertainties of 100K and 3-4%, respectively. V1598Cyg consistently exhibitsspots that are a few hundred degrees warmer than the photosphere, most likelyindicating that the light curve is dominated by chromospheric plage. The spotactivity varies over our observing baseline, with a typical time scale of0.5-1yr, which we interpret as the typical spot lifetime. Combining our lightcurve with archival data, we find a six year cycle in the average brightness,that is probably a sign of a magnetic activity cycle.

A survey for variable young stars with small telescopes: III -- Warm spots on the active star V1598Cyg