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Optical Flare Search on the RS CVn-type flare stars AR Lacertae

We present the results of fast spectrophotometry of flares on the RS CVn-type star AR Lac with a time resolution of 34 s and a spectroscopic resolution R $\sim $ 1300. The observations were performed on July 21-22, 2021 with the 2.0 m Karl Zeiss telescope at the Terskol Observatory. During the flares, an additional emission appeared in the spectrum of AR Lac at wavelengths nearby Ca II H\&K ($λ$ = 3933, 3968 Å), nearby Si IV lines ($λ$ = 4089, 4116 Å). Variations in these lines range from 2 to 5\%. We have estimated the UBV magnitudes from spectrograms through a mathematical convolution of the spectra with the filter transmission curves. The flare amplitudes in the UBV band up to 0.8 magnitudes were discovered. A detailed colorimetric analysis has allowed important parameters of the flares on AR Lac to be estimated: the temperatures at maximum light and their sizes. The color-color $(U-B)$ - $(B-V)$ diagrams confirm that all the flares at maximum light radiate as a blackbody. The temperatures at maximum light were up to 12000 $\pm $ 300 K. Based on our colorimetric analysis, we have estimated the linear sizes of the flares at maximum light. The linear size of the flares at the maximum luminosity is approximately 2\% of the radius of the star.