Abstract
We consider nonperturbative four-dimensional [Formula: see text] space–time supersymmetric orientifolds corresponding to Type I compactifications on (generalized) Voisin–Borcea orbifolds. Some states in such compactifications arise in "twisted" open string sectors which lack world sheet description in terms of D-branes. Using Type I-heterotic duality as well as the map between Type IIB orientifolds and F theory we are able to obtain the massless spectra of such orientifolds. The four-dimensional compactifications we discuss in this context are examples of chiral [Formula: see text] supersymmetric string vacua which are nonperturbative from both orientifold and heterotic points of view. In particular, they contain both D9- and D5-branes as well as nonperturbative "twisted" open string sector states. We also explain the origins of various inconsistencies arising in such compactifications for certain choices of the gauge bundle.
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