  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 {\it 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 correponding
low-energy effective lagrangians. In particular we show how the exact moduli
dependence of some F^4 terms in the eight-dimensional type II (4,0) orbifold is
reproduced by the infinite sum of D-instanton contributions in the dual type I
theory.
