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Published online by Cambridge University Press: 27 September 2017
On the basis of 11 years of UBV photometry the light and color variability of the long-period RS CVn binary HK Lac (KOIII) is discussed. From the U-B data, which reflect the chromospheric activity, a quasi-periodic variation with the cycle length of about 5.5 years is suspected. Less evidently, a 2.7 years long overall spot coverage cycle is seen from the V(max) data.
No cyclic variation is found from the B-V data representing the photospheric temperature conditions, although there is an indication that the star was the reddest around 1985, when the chromospheric activity was in maximum.
At that time, in late 1985, we observed the star on 6 nights in UBV color in order to detect short-term variability. Fluctuations up to 0.03 magn. on time scales of 20-30 minutes were likely present. The mplitude is the highest in U colour.