Non-perturbative K3 orientifolds with NS-NSB-flux

We consider non-perturbative six dimensional N = 1 space-time supersymmetric orientifolds of Type IIB on K3 with non-trivial NS-NS B-flux. All of these models are non-perturbative in both orientifold and heterotic pictures. Thus, some states in such compactifications arise in "twisted" open string sectors which lack world-sheet description in terms of D-branes. We also discuss their dual F-theory compactifications on certain Voisin-Borcea orbifolds. In particular, the explicit construction of non-perturbative K3 orientifolds with NS-NS B-flux gives additional evidence for the conjectured extension of Nikulin's classification in the context of Voisin-Borcea orbifolds. (C) 1999 Published by Elsevier Science B.V. All rights reserved.