class of explicit orbifold compactifications of the heterotic string for the standard (or “Type I”) see-saw in its minimal form. The fact that no such viable mechanism was found may suggest that often-neglected alternatives to the standard see-saw may have more theoretical motivation than considerations of simplicity, elegance, or GUT structure would otherwise indicate.

In this work we study the properties of a new construction of see-saw mechanisms that is motivated by known string constructions. The mechanism is an example of the Higgs triplet or “Type II” seesaw [@Lazarides:1980nt; @Mohapatra:1980yp; @Schechter:1981cv; @Ma:1998dx; @Hambye:2000ui; @Rossi:2002zb], but the stringy origin has important implications for the mixings and mass hierarchy that distinguish it from conventional “bottom-up” versions of the triplet model. After outlining the model in a general way below we will motivate its plausibility in string theory by considering a particular $\mathbb{Z}_3 \times
\mathbb{Z}_3$ orbifold of the heterotic string [@Font:1989aj], where several of the properties needed for a fully realistic model are manifest.

General Features of $SU(2)$ Triplet Models {#sec:general}
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Let us briefly review the form of the effective neutrino mass matrix to establish our notation and to allow the contrast between models involving triplets of $SU(2)_L$ and those involving singlets 