The RS CVn binary HK Lacertae : long-term photometry from Sonneberg sky-patrol plates

Long-term photographic photometry of the active long-period RS CVn binary HK Lac (HD 209813) was obtained from more than 2000 Sonneberg Sky-Patrol plates taken between 1956 and 1996. We achieve an internal accuracy of 0m07. The correspondence with contemporaneous high-precision photoelectric photometry from automatic telescopes is striking and successfully demonstrates the feasibility of our approach. Based on a Bayesian time series analysis, we improve the previously published cycle period to 13.37 ± 0.08 years, and present evidence of an additional period of 9.48 ± 0.13 years. This establishes the multi-periodicity of dynamo action in these overactive stars as compared to the Sun. The already known 6.7-years cycle turns out to be an overtone of the dominating 13.4-years cycle. Our long-term photographic photometry even allowed the detection of the star's mean rotational period of 24.35 days.