ULTRAVIOLET AND OPTICAL FLARES ON GL-866

We present simultaneous ultraviolet and optical observations of five f lares on the very late type M dwarf Gl 866 (dM5.5e). A procedure to es timate the physical parameters of the flaring plasma has been develope d using a simplified model of the flare and a comparison of observed a nd computed Balmer decrements. With this procedure we have determined the optical thickness, electron temperature and electron density of th e flaring plasma at flare maximum for three spectroscopically observed flares. The three spectroscopically observed flares cover up to 5 per cent of the stellar surface and have areas similar to solar flares.