SIMBAD references

We present the result of the Hα monitoring of the long-period RS CVn-type binary HK Lac carried out in the fall of 1989. Although the observations are not well distributed along the rotation period, the rotational modulation of the Hα equivalent width shows up with a maximum around phase 0.2, i.e. about at the same longitude as the large spotted area found by Olah et al. (1991) from photometric observations. The more relevant result is the observation of the complete development of a strong flare lasting about six days. The total energy, released during the event in the Hα line, is estimated to be about 1.3x10³⁷ erg. We show that the binary nature of the star does not play a significant role in the energetics of the flare. The amount of released energy is consistent with that stored in a magnetic structure whose dimension is about one half that of the active secondary component. We remark that the flare occurred when a new spotted area developed on the visible hemisphere as reported by Olah et al. (1991). This gives, for the first time, evidence for a stellar flare eruption resulting from the interaction between an old and a new magnetic structure as often observed in the Sun (Gaizauskas 1989). The occurrence of a similar event in 1977 is discussed.