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Astron. Astrophys. 364, 641-645 (2000)

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1. Introduction

The first quantitative characteristics of optical flares of the UV Ceti-type red dwarf variables consisted of estimations of their amplitude and duration. Subsequent photoelectric monitoring gave more detailed data on the whole light curve for such events. However, attempts to represent these more complete light curves within the framework of different physical and formal mathematical models did not lead to any understanding of the events which caused them (Gershberg 1970; Coluzzi et al. 1978). Investigators have examined the following characteristic features of light curves of stellar flares: fast brightness decays just after flare maxima (Shakhovskaya 1974), preflare dips in stellar brightness (Cristaldi et al. 1980), the second maximum of flare brightness (Katsova & Livshits 1992). It should be noted that when analyzing the solar flares, one can distinguish beginning impulse phases and subsequent thermal gradual phases of flares. Amongst the vast variety of stellar flare light curves, a special place is occupied by the flare in the Hyades flare star HII 2411, which occured on the 20th November 1972 and was detected by Rodonó (1974). During this rather intense flare, he registered highly oscillatory phenomena with the oscillation period varying from 13.7 to 12.4 s throughout the recording time of about 12 min; the oscillation amplitude appeared to be constant and was equal to about 25% of the mean intensity in the quiescent phase in white light. As was noted by Rodonó, the observed rapid oscillations were not visible during the out-of-flare phases or during the development of other, smaller flares. For the next quarter of a century, no further similar events were observed. Here, for the first time, we describe similar optical brightness pulsations in the EV Lac flares which we detected with a more accurate technique including many-site multichannel monitoring and which we analyzed with much more sophisticated methods.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001