Non-Perturbative effects from orbifold constructions
  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. 
G. Aldazabal