Long-term photographic photometry of HK Lac

We achieve an internal accuracy of 0.07mag. 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 periodogram analysis, we improve previously published cycle periods to 13.37+/-0.8 and 6.7+/-0.1-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. Of course, the 6.7-years cycle may be an overtone of the dominating 13.4-years cycle.