Lepton-Pair Production in Virtual Compton Scattering off the Proton

We show that lepton-pair production in Virtual Compton Scattering offers, through interference with the well-known Bethe-Heitler process, a sensitive probe to learn the longitudinal response of resonances and the electromagnetic nucleon form factors. This interference can be measured directly in terms of an asymmetry. The role of off-shell effects in the N-N-$\gamma$ vertices is investigated as well. An additional N-N-$\gamma -\gamma$ contact term in the amplitude, included to ensure gauge invariance of the model, cancels a substantial part of the off-shell effects.