Fast colorimetry of the flare star EV lacertae from UBVRI observations in 2004

Aims. We report results of a quantitative colorimetric UBVRI analysis of two flare events on the red dwarf EV Lac. The photometric data were obtained in September 2004, during the multi-site synchronous monitoring from the four observatories in Ukraine, Russia, Greece, and Bulgaria. These observations confirmed the presence of small-scale high-frequency oscillations (HFO) initially detected by Rodonó (1974, A&A, 32, 337) and recently reconfirmed by the authors. Here we discuss the color characteristics of flares and HFO. Methods. Colorimetric analysis had been performed with the help of the time tracks in the UBVRI color-color diagrams from the earliest phase of flare development. Digital filtering technique was used to evaluate the time-dependent color indices. Results. As can be clearly seen in the diagrams, color indices oscillate on a time scale of seconds, far exceeding instrumental errors. Regarding the HFO, we conclude that the bulk of a flare oscillates during a major part of its lifetime between the states of hydrogen plasma opaque and transparent in the Balmer continuum. We find that at the peaks of oscillations the color tracks drift into the regions of color-color diagrams corresponding to a blackbody radiation, which provides an estimate of color temperatures from 17000 to 22 000 K. We also find that flares cover ∼ 1 % of the stellar disc.