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Detection of high-frequency variability in chromospherically active stars

We have carried out high-speed photometry of three chromospherically active stars, BD +15 3364, II Peg, and SAO 52355 with the Zeiss 2-m telescope, as well as low-resolution spectroscopy of SAO 52355 with the Zeiss-600 telescopes at Peak Terskol. BD +15 3364 is known as chromospherically active star of the spectral type G0. II Peg is the RS CVn binary. It has a spectral type K2IV-Ve. SAO 52355 is a field star of the spectral type K0 III. The X-ray observations taken by Ginga and ROSAT indicate in II Peg and SAO 52355 coronae plasma temperatures as high as $10^{7}$ K. These two stars are supposed to have high-powered chromospheres. Photometric observations of all three stars show high-frequency variations in brightness in the UBV bands at subsecond range. Intensity variations are found peaked at frequency around about 0.5 Hz, spanning the range up to 1.5 Hz for BD +15 3364, II Peg, and up to about 35 Hz for SAO 52355. The relative power of fluctuations reaches ($10^{-3.7} - 10^{-4.2}$) in the UBV bands. Spectroscopic monitoring of SAO 52355 showed variations of emission in the Balmer lines and in the CaII H, K lines at time intervals ranging from seconds to minutes. From the power spectrum data one can find that variations in the intensities of the CaII H, K and $H_γ$ lines are 3.2% and 1.5%, respectively. This allows us to assert the existence of intense microflaring activity in these stars.