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We report on time series photometric observations of the dM4.5e flare star YZ CMi taken in 1993 Nov with the High Speed Photometer aboard the Hubble Space Telescope . The data consist of five 30 minute time sequences, with a sampling rate of 0.01s, which were taken through the F240W filter (centered at 240nm with a 800 \AA\ width). This setup was selected since the flare energy is expected to be large and the stellar background is small at these wavelengths. The observations show a stellar background of 120 counts s$^{-1}$ on which are superimposed well defined flare events ranging in integrated energy from 10$^{28}$ to 10$^{30}$ ergs, as well as longer term variations with an amplitude of up to 50\% of the continuum intensity and time scales ranging from several minutes to hours. These results are compared with data from the dM8e flare star CN Leo, obtained earlier with the same experimental setup. CN Leo has a quiescent X-ray flux which is 1/30 that of YZ CMi, but had a higher occurrence rate of microflares. On the other hand, the stellar UV background, which must be chromospheric in origin, is less than 1/25th that seen on YZ CMi. This suggests a link between chromospheric and coronal heating.
