Resonant neutrino transitions and nucleosynthesis

We consider resonant transitions of the electron neutrino ve into an inert neutrino species vx at the onset of the primordial nucleosynthesis. The neutron-to-proton scattering rates will be modified due to these transitions. We show that the observed helium abundance puts limits on the possible neutrino mixing parameters which are more stringent by several orders of magnitude as compared with reactor experiment bounds. (orig.)