Neutrino Oscillations from Dirac and Majorana Masses
  We present a scenario of neutrino masses and mixing angles. Each generation includes a sterile right handed neutrino in addition to the usual left handed one. We assume a hierarchy in their Dirac masses similar to, but much larger than the hierarchies in the quarks and charged leptons. In addition, we include a Majorana mass term for the sterile neutrinos only. These assumptions prove sufficient to accomodate scales of mass differences and mixing angles consistent with all existing neutrino oscillation data. 
D. Ring