Fast UBVRI colorimetry of stellar flares on EV Lacertae

Here we report first results of colorimetric analysis of the flare star EV Lac based on the UBVRI data obtained during the course of many-site synchronous monitoring in 1998 September. The color characteristics of flares and small-scale high-frequency oscillations (HFO) recently discovered are in the focus of this work. Colorimetric analysis had been performed with the help of color time tracks, as from the earliest phase of the flare development. This technique is illustrated by theexample of a relatively strong flare event of EV Lac on September 11, 1998. The digital filtering technique was used to evaluate: 1) the color indices of both the main flare light curves and HFO, 2) their time behavior during the flare lifetime. It is found that the color indices of the main outburst light curves differ significantly from those of rapid oscillations. The first shows smooth forms. In contrast to this, the corresponding HFO indices are greatly variable, in particular for the ascending part of a burst. On account of HFO the flare color oscillates from deep red to extremely blue on a time scale of seconds. When the flare grew to Delta U ≃ - 0.8 mag, color tracks were evolved into the region of color-color diagrams occupied by hydrogen plasma which is optically thick at the Balmer continuum.