  A Double Field Theory (DFT) description of gauge symmetry enhancing-breaking
in the heterotic string is presented. The construction, based on previous
results for the bosonic string, relies on the extension of the tangent frame of
DFT. The fluxes of a Scherk-Schwarz like generalized toroidal compactification
are moduli dependent and become identified with the structure constants of the
enhanced group at fixed "self-dual" points in moduli space. Slight
displacements from such points provide the breaking of the symmetry, gauge
bosons acquiring masses proportional to fluxes. The inclusion of fermions is
also discussed.
