String-Inspired Triplet See-Saw from Diagonal Embedding of SU(2)_L in   SU(2)_A x SU(2)_B
  Motivated by string constructions, we consider a variant on the Type II see-saw mechanism involving the exchange of triplet representations of SU(2)_L in which this group arises from a diagonal embedding into SU(2)_A x SU(2)_B. A natural assignment of Standard Model lepton doublets to the two underlying gauge groups results in a bimaximal pattern of neutrino mixings and an inverted hierarchy in masses. Simple perturbations around this leading-order structure can accommodate the observed pattern of neutrino masses and mixings. 
Paul Langacker and Brent D. Nelson (University of Pennsylvania)