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Astron. Astrophys. 334, 931-934 (1998)

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1. Introduction

The well-known flare star EV Lac shows a wide variety of activity phenomena at different time scales. Several hundreds of classical solar-like flares were registered on it by a number of observers. However, the classical flares are not such a frequent phenomenon for EV Lac as one uses to think. As mentioned by Roizman and Kabitchev (1985), a considerable fraction of flares shows a complicated multipeak structure with sudden onset and decay. Periodic light variations with a period of 4.378 days and an amplitude 0.07 in V-magnitude were reported by Pettersen (1980) as caused by starspots and rotation. New data on this kind of variations were summarized recently by Contadakis (1995) who indicates that a long-term restructuring of the starspot areas takes place on EV Lac. Careful analysis of the quiet-state luminosity variations of the star showed the presence of occasional short-period oscillations as well as of irregular variations on a time basis of 2 hours with an amplitude of a few tenths of a magnitude (Mavridis and Vargolis, 1993). In recent years, a few reports appeared about observations of short-time outbursts in EV Lac with a duration of 0.1 to several seconds. These results were summarized briefly by Zhilyaev (1994). The history of flare stars photometry yields many examples of a pre-flare dip phenomenon. The first pre-flare dips observations in YZ CMi were reported by Rodono et al. (1979) and confirmed by Cristaldi et al. (1980). They are characterized by an amplitude of a few tens of percent in optical bands and durations of a few seconds. A low amplitude pre-flare dip with an exceptionally long duration of 36 min was detected in the light curve of V1054 Oph by Ventura et al. (1994). Recent results on this topic were summarized by Ventura et al. (1995). A new kind of flares in EV Lac was reported by Zhilyaev and Verlyuk (1995). A few events showed a flare in the U band accompanied by a dip in the V band at the same time with a duration of a few seconds and fading amplitudes of a few hundredths. To clarify the situation international campaigns for simultaneous observations of EV Lac were carried out in the years 1995-96 at three Observatories in Ukraine, Russia and Bulgaria. The results of these campaigns are reported here.

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© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998