the Type II variety [@King:2003jb; @DeGouvea:2005gd; @Langacker:2004xy; @Altarelli:2004za; @Mohapatra:2004vr] would typically retain the right-handed neutrinos and utilize all the components in the mass matrix of (\[general\]) to explain the neutrino masses and mixings. These examples are often inspired by SO(10) GUT considerations or are couched in terms of left-right symmetry more generally. The latter commonly employ additional Higgs fields transforming as $(1,\mathbf{3})$ under $SU(2)_L
\times SU(2)_R$, which acquire vevs to break the gauge group to the Standard Model.

Instead we imagine a process by which the $SU(2)$ of the Standard Model emerges as the result of a breaking to a [*diagonal*]{} subgroup $SU(2)_A \times SU(2)_B \to SU(2)_L$ at a very high energy scale. Furthermore, while we will employ two conjugate triplet representations which form a vector-like pair under the Standard Model gauge group with $Y = \pm 1$, we do not seek to embed this structure into a left-right symmetric model.

Diagonal Embedding of $SU(2)_L$ {#sec:diagonal}
===============================

In attempting to embed the framework of the previous section in a model of the weakly coupled heterotic string we immediately encounter an obstacle: the simplest string constructions contain in their massless spectra only chiral superfields which transform as fundamentals or (anti-fundamentals) of 