Maximal mixing neutrino models
  We account for the solar and atmospheric neutrino problems by introducing maximal mixing between conventional and sterile neutrino partners. We achieve this by invoking a seesaw-like mechanism which not only provides us with maximally mixed neutrino/sterile-neutrino mass eigenstates but also accounts for the relative suppression of the neutrino masses compared to the charged fermion masses. In obtaining such an extended seesaw mechanism we are required to introduce a new U(1) global symmetry together with an extended Higgs sector. 
J P Bowes and R R Volkas