Diagnostics of Inverse-Compton models for the gamma-ray emission of 3C 279 and Mkn 421

We discuss the radio to gamma-ray emission of 3C 279 and Mkn 421 using the same simple model of a homogeneous region emitting via the synchrotron and Inverse-Compton mechanisms. Synchrotron photons or, alternatively, photons outside the jet are considered as seeds for inverse Compton scattering for 3C 279. Both models can reproduce the overall spectra of 3C 279, but we stress that simultaneous optical-gamma-ray monitoring can discriminate between the two alternatives. For Mkn 421, the lack of emission lines and the extremely high electron energies responsible for the TeV emission favour the synchrotron self Compton model.