Abstract
Consistent heterotic orbifold vacua, includingnonperturbative information, can be constructed.Generically, the usual modular invariance constraintsare violated and thus, anomalies are expected. However, five-branes do appear in the correct proportionjust to ensure the consistency of the full theory. Wefirst exemplify the situation in six dimensions, N = 1,where strong-coupling effects, implying the presence of five-branes, are better known.Explicit N = 1 models in D = 4, essentially inheritedfrom six dimensions, are then presented. In particularwe show concrete examples of models which exhibit nonperturbative transitions leading to gaugeenhancement and/or to a change in the number of chiralgenerations.
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