  Stellar flares affect all atmospheric layers from the photosphere over
chromosphere and transition region up into the corona. Simultaneous
observations in different spectral bands allow to obtain a comprehensive
picture of the environmental conditions and the physical processes going on
during different phases of the flare. We investigate the properties of the
coronal plasma during a giant flare on the active M dwarf CN Leo observed
simultaneously with the UVES spectrograph at the VLT and XMM-Newton.From the
X-ray data, we analyze the temporal evolution of the coronal temperature and
emission measure, and investigate variations in electron density and coronal
abundances during the flare. Optical Fe XIII line emission traces the cooler
quiescent corona. Although of rather short duration (exponential decay time < 5
minutes), the X-ray flux at flare peak exceeds the quiescent level by a factor
of 100. The electron density averaged over the whole flare is greater than 5 *
10^11 cm^-3. The flare plasma shows an enhancement of iron by a factor of
approx. 2 during the rise and peak phase of the flare. We derive a size of
<9000 km for the flaring structure from the evolution of the the emitting
plasma during flare rise, peak, and decay. The characteristics of the flare
plasma suggest that the flare originates from a compact arcade instead of a
single loop. The combined results from X-ray and optical data further confine
the plasma properties and the geometry of the flaring structure in different
atmospheric layers.
