We report the results of a high temporal resolution spectroscopic monitoring of the flare star AD Leo .
During 4 nights , more than 600 spectra were taken in the optical range using the Isaac Newton Telescope ( INT ) and the Intermediate Dispersion Spectrograph ( IDS ) .
We have observed a large number of short and weak flares occurring very frequently ( flare activity > 0.71 hours ^ { -1 } ) .
This is in favour of the very important role that flares can play in stellar coronal heating .
The detected flares are non white-light flares and , though most of solar flares belong to this kind , very few such events had been previously observed on stars .
The behaviour of different chromospheric lines ( Balmer series from H \alpha to H _ { 11 } , Ca ii H & K , Na i D _ { 1 } & D _ { 2 } , He i 4026 Å and He i D _ { 3 } ) has been studied in detail for a total of 14 flares .
We have also estimated the physical parameters of the flaring plasma by using a procedure which assumes a simplified slab model of flares .
All the obtained physical parameters are consistent with previously derived values for stellar flares , and the areas – less than 2.3 \% of the stellar surface – are comparable with the size inferred for other solar and stellar flares .
Finally , we have studied the relationships between the physical parameters and the area , duration , maximum flux and energy released during the detected flares .