Starspot modelling of the RS CVn binary HK lacertae

More than 270 nightly mean magnitudes of the long-period RS CVn binary HK Lac are investigated, and conclusions are drawn about the nature of its complicated light variations. The mean brightness, the apparent photometric period, and the shape of the light curve all show strong variations. Analysis with a starspot model assuming two large spots and a general uniform spottedness indicates two comparably large spots which appear to have maintained their separate identities for the last 15 yr and drifted in longitude separation from each other smoothly by only about 45 deg. The phase of the two spots indicates both are rotating very nearly synchronously with the orbital motion, one slightly (0.025-percent) faster and the other slightly (0.080-percent) slower. The latitudes of the two spots, one farther above the equator and one closer to the equator, are consistent with solar-type differential rotation and yield an estimate of 25 + or - 12 deg for the corotating latitude. A correlation between mean spot latitude and instantaneous photometric period yields another estimate of 31 + or - 2 deg, in agreement with the first.