Abstract

We consider D = 6, N = 1, Z M orbifold compactifications of heterotic strings in which the usual modular invariance constraints are violated. It is argued that in the presence of non-perturbative effects many of these vacua are nevertheless consistent. The perturbative massless sector can be computed explicitly from the perturbative mass formula subject to an extra shift in the vacuum energy. The non-perturbative piece is given by five-branes either moving in the bulk or stuck at the fixed points. We also discuss how to carry out this type of construction to the D = 4, N = 1 case and specific examples are presented in which non-perturbative transitions changing the number of chiral generations do occur.

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