Cool Stars 14 - Submitted Abstract # 17 This version created on 05 October 2006 Simultaneous V-band and soft X-ray observation of a giant flare on an M8 dwarf Beate Stelzer, Osservatorio Astronomico di Palermo Giuseppina Micela, Osservatorio Astronomico di Palermo Juergen Schmitt, Hamburger Sternwarte We present simultaneous V-band and soft X-ray observations of a giant flare on the M8 star LP412-31. The data was obtained with XMM-Newton's Optical Monitor and X-ray CCD camera. With an increase of the V band count rate corresponding to ~6 mag, and a similar amplitude (factor of ~100) of the X-ray lightcurve, this is one of the most outstanding events throughout the whole history of stellar flare observations. Its relevance is further bolstered by the extremely cool spectral type of the host star. The flare is of the impulsive type (decay time shorter than 1 hour), and covers only 20% of the observing time. Outside the flare the star was inactive, displaying constant and very weak X-ray and V band emission. In contrast, X-ray observations of most higher-mass active stars have shown continuous low-level variations, suggesting a power-law distribution of flare amplitudes. These differences may indicate qualitative changes in the character of magnetic activity in the predominantly neutral atmospheres of the coolest stars and brown dwarfs. Using timing and spectral analysis we examine the properties of the emitting plasma in the context of standard flare models. ----------------------------------