  We examine to what extent heterotic string worldsheets can describe arbitrary
E8xE8 gauge fields. The traditional construction of heterotic strings builds
each E8 via a Spin(16)/Z2 subgroup, typically realized as a current algebra by
left-moving fermions, and as a result, only E8 gauge fields reducible to
Spin(16)/Z2 gauge fields are directly realizable in standard constructions.
However, there exist perturbatively consistent E8 gauge fields which can not be
reduced to Spin(16)/Z2, and so cannot be described within standard heterotic
worldsheet constructions. A natural question to then ask is whether there
exists any (0,2) SCFT that can describe such E8 gauge fields. To answer this
question, we first show how each ten-dimensional E8 partition function can be
built up using other subgroups than Spin(16)/Z2, then construct ``fibered WZW
models'' which allow us to explicitly couple current algebras for general
groups and general levels to heterotic strings. This technology gives us a very
general approach to handling heterotic compactifications with arbitrary
principal bundles. It also gives us a physical realization of some elliptic
genera constructed recently by Ando and Liu.
