Majorana and Dirac masses, neutrino oscillations and the number of charged leptons

A scheme of lepton mixing is proposed in which the number of the charged leptons, as well as the number of neutrino types must be even. The Dirac and the Majorana masses coexist and a "hierarchy' of different oscillation types is quite natural (for example electron neutrino reversible muon neutrino oscillations might be suppressed with respect to electron neutrino reversible vsub(tau) oscillations). The scheme requires at least one more charged lepton and one more neutrino in addition to the well known electron, muon tau electron neutrino, muon neutrino and neutrino sub(tau) leptons. In the Lagrangian the Dirac and Majorana mass terms are present on equal footing. The existence of one pair of quarks in addition to the well known u, d, s, c and b(t) quarks is required too.