4-DIMENSIONAL CHIRAL N=1 TYPE-I VACUA WITH AND WITHOUT D5-BRANES

In this paper we consider compactifications of type I strings on Abeli an orbifolds. We discuss the tadpole cancellation conditions for the g eneral case with D9-branes only. Such compactifications have (perturba tive) heterotic duals which are also realized as orbifolds (with non-s tandard embedding of the gauge connection), The latter have extra twis ted states that become massive once orbifold singularities are blown u p. This is due to the presence of a perturbative heterotic superpotent ial with couplings between the extra twisted states, the orbifold blow -up modes, and (sometimes) untwisted matter fields. Anomalous U(1) (ge nerically present in such models) also plays an important role in type I-heterotic (tree-level) duality matching. We illustrate these issues on a particular example of Z(3) x Z(3) orbifold type I model (and its heterotic dual). The model has N = 1 supersymmetry, U(4)(3) x SO(8) g auge group, and chiral matter. We also consider compactifications of t ype I strings on Abelian orbifolds with both D9- and D5-branes. We dis cuss tadpole cancellation conditions for a certain class of such model s. We illustrate the model building by considering a particular exampl e of type I theory compactified on Z(6) orbifold, The model has N = 1 supersymmetry, [U(6) x U(6) x U(4)](2) gauge group, and chiral matter. This would correspond to a non-perturbative chiral vacuum from the he terotic point of view. (C) 1998 Elsevier Science B.V.