NONPERTURBATIVE HETEROTIC D=6, D=4, N=1 ORBIFOLD VACUA

We consider D = 6, N = 1, Z(M) orbifold compactifications of heterotic strings in which the usual modular invariance constraints are violate d. 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 s ubject to an extra shift in the vacuum energy. This shift is associate d to a non-trivial antisymmetric B-field flux at the orbifold fixed po ints. The non-perturbative piece is given by five-branes either moving in the bulk or stuck at the fixed points, giving rise to Coulomb phas es with tensor multiplets. The heterotic duals of some type IIB orient ifolds belong to this class of orbifold models, We also discuss how to carry out this type of construction to the D = 4, N = 1 case and spec ific Z(M) X Z(M) examples are presented in which non-perturbative tran sitions changing the number of chiral generations do occur. (C) 1998 E lsevier Science B.V.