Leptonic decay of heavy flavors

The spectrum of charged leptons in semileptonic B and D decays is reanalyzed. Special emphasis is given to the behaviour near the end point which is of importance for the determination of the b -> u versus b -> c couplings. In particular the effects of soft gluons are studied and their contribution is resummed in leading double logarithmic approximation. This effect determines the end-point behaviour for the decay of a heavy quark into massless quark and leptons, such as b -> u e-anti νsub(e). Bound-state corrections to the parton picture are treated in a model which satisfies all kinematical constraints. A comparison of predictions for charm decay with experimental data is also presented. (orig.)