  A QCD analysis of the reactions A B -> gamma^* gamma and gamma^* gamma -> A B
in the forward region shows that they can be factorized in a way which is quite
similar to the frameword developped for deeply virtual Compton scattering. The
generalized parton distributions (GPDs) related to this latter process being
replaced by new non perturbative hadronic matrix elements, the transition
distribution amplitudes (TDAs).
