[en] The transfer of angular momentum between the bar and halo components of a galaxy is computed using both analytic dynamical friction theory and 'semi-restricted' n-body simulations. The two methods yield results which are in qualitative agreement and demonstrate that dynamical friction can exert strong torques on galactic bars. A rotating rigid bar with typical parameters initially ending at corotation slows down in several bar rotation times in the presence of an isothermal halo. This rapid angular momentum transfer implies that the corotation radius must be far beyond the end of the bar unless: (1) angular momentum is added to the bar (i.e. by the disc) allowing an equilibrium bar pattern speed to be reached; (2) the galactic halo is small or non-existent; or (3) the bar is weak. (author)