Context : 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 .

Aims : 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 .

Methods : 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 .

Results : Although of rather short duration ( exponential decay time \tau _ { LC } < 5 minutes ) , the X-ray flux at flare peak exceeds the quiescent level by a factor of \approx 100 .
The electron density averaged over the whole flare is greater than 5 \cdot 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 .

Conclusions : 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 .