D-strings in unconventional type I vacuum configurations

We determine the spectrum of D-string bound states in various classes of generalized type I vacuum configurations with sixteen and eight supercharges. The precise matching of the BPS spectra confirms the duality between unconventional type IIB orientifolds with quantized NS-NS antisymmetric tensor and heterotic CHL models in D = 8. A similar analysis puts the duality between type II (4,0) models and type I strings without open strings on a firmer ground. The analysis can be extended to type II (2,0) asymmetric orbifolds and their type I duals that correspond to unconventional K3 compactifications. Finally we discuss BPS-saturated threshold corrections to the corresponding low-energy effective Lagrangians. In particular we show how the exact moduli dependence of some F-4 terms in the eight-dimensional type IT (4,0) orbifold is reproduced by the infinite sum of D-instanton contributions in the dual type I theory. (C) 1999 Elsevier Science B.V.