A new contribution to the flavour-changing lepton- photon vertex

We show that the correct perturbation theory for mixed fermion states leads to nonvanishing contributions of the dimension-four vertex operators for flavour-changing transitions. Their contributions to the amplitude are of the same order of magnitude as the dimension- five vertex operators. Considerations are valid irrespective of the electroweak model. A complete and correct evaluation of the neutrino lifetime for particular electroweak models could be of greate importance in the theoretical cosmology and astrophysics.