  We indicate how consistent heterotic orbifold compactifications, including
non perturbative information, can be constructed. We first analyse the
situation in six dimensions, N=1, where strong coupling effects, implying the
presence of five branes, are better known. We show that anomaly free models can
be obtained even when usual modular invariance constraints are not satisfied.
The perturbative massless sector can be computed explicitly from the
perturbative mass formula subject to an extra shift in the vacuum energy.
Explicit examples in D=4, N=1 are presented. Generically, examples exhibit non
perturbative transitions leading to gauge enhancement and/or changing the
number of chiral generations.
