  We discuss the compatibility of the most accurate experimental data with the
ordinary Migdal-Eliashberg theory of superconductivity in the fullerene
compound Rb_3C_60. By using different model phonon spectra we conclude that the
experimental data can be fitted only by invoking an electron-phonon coupling of
order \lambda\simeq 3. This exceedingly high value is unphysical and it is not
consistent with the basic assumptions of the Migdal-Eliashberg theory. On the
contrary, by relaxing the adiabatic hypothesis on which the Migdal-Eliashberg
theory rests, the experimental data can be fitted by much more realistic values
of \lambda. This generalized theory predicts also characteristic features
absent in the Migdal-Eliashberg framework which can be experimentally tested.
