Form-factors of semi-leptonic decays of heavy mesons

Semi-leptonic decays of quarkonia to quarkonia (e.g. B _c → J/Ψ) offer a unique possibility to investigate the structure of confined systems. A QCD sum rule analysis yields a momentum transfer dependence which is much stronger than predicted by vector meson dominance. It is argued that this is a typical reflection of the bound state nature and its experimental investigation could give important insights into the nature of bound states in a confining theory.