  We investigate the effect of stellar activity and flares on short-term radial
velocity measurements in the mid-M flare star CN Leo. Radial velocity
variations are calculated from 181 UVES spectra obtained during three nights.
We searched for spectral orders that contain very few atmospheric absorption
lines and calibrated them against the telluric A-band from O$_2$ in the Earth's
atmosphere. One giant flare occurred during our observations, which has a very
strong effect on radial velocity. The apparent radial velocity shift due to the
flare is several hundred m s$^{-1}$ and clearly correlated with H$\alpha$
emission. Outside the flare, only spectral orders containing the most prominent
emission lines of H, He, and Ca show a correlation to chromospheric activity
together with a radial velocity jitter exceeding a few 10 m s$^{-1}$. We
identify a number of spectral orders that are free of strong emission lines and
show no flaring-related radial velocity jitter, although flares occurred as
strong as 0.4 dex in normalized H$\alpha$ luminosity. The mean radial velocity
jitter due to moderate flaring is less than 10 m s$^{-1}$. Strong flares are
easily recognized directly in the spectra and should be neglected for planet
searches.
