  Cool stars are known to produce flares probably as a result of magnetic
reconnection in their outer atmospheres. We present simultaneous XMM-Newton
optical V band and X-ray observations of the M8 dwarf LP412-31. During the
observation a giant flare occurred, with an optical amplitude of 6 mag and
total energy of 3 10^(32) erg in each the V band and soft X-rays. Both flare
onset and flare decay were completely covered in both wavebands with a temporal
resolution of 20 s, allowing for a determination of the flare time scales as
well as a study of the temperature evolution of the flaring plasma. The data
are consistent with an impulsive energy release followed by radiative cooling
without further energy release during the decay phase. Our analysis suggests
that the optical flare originates from a small fraction of the surface of
LP412-31, while the characteristic scale size of the flaring X-ray plasma is of
the order of the stellar radius or larger. The absence of any small-scale
variability in the light curve suggests a non-standard flare number energy
distribution.
