Non-perturbative orientifolds

We consider non-perturbative six and four dimensional N = 1 space-time supersymmetric orientifolds. 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 we are able to obtain the massless spectra for some of such orientifolds. The four dimensional compactification we discus in this context is an example of a chiral N = 1 supersymmetric string vacuum which is non-perturbative in both orientifold and heterotic pictures. In particular, it contains both D9- and D5-branes plus non-perturbative "twisted" open string sector states as well. (C) 1999 Published by Elsevier Science B.V. All rights reserved.