See-Saw Type Mixing and $ \nu_{\mu} \to \nu_{\tau} $ Oscillations

We consider $\nu_{\mu} \to \nu_{\tau}$ oscillations under the assumption that there is a see-saw type mixing of the light neutrinos with heavy Majorana particles. It is shown that the existing data, including the recent LEP data, do not exclude the possibility that the additional terms in the transition probability due to this mixing could be of the same order of magnitude as the usual oscillating term. Detail investigations of $\nu_{\mu} \to \nu_{\tau}$ transitions in future CERN and Fermilab experiments could allow to get informations not only about the neutrino masses and mixing but also about the mixing of neutrinos with heavy Majorana particles.