Orientifolds of string theory Melvin backgrounds

We study the dynamics of type I strings on Melvin backgrounds, with a single or multiple twisted two-planes. We construct two inequivalent types of orientifold models that correspond to (noncompact) irrational versions of Scherk-Schwarz type breaking of supersymmetry. In the first class of vacua, D-branes and O-planes are no longer localized in space-time but are smeared along the compact Melvin coordinate with a characteristic profile. On the other hand, the second class of orientifolds involves O-planes and D-branes that are both rotated by an angle proportional to the twist. In case of "multiple Melvin spaces", some amount of supersymmetry is recovered if the planes are twisted appropriately and part of the original O- planes are transmuted into new ones. The corresponding boundary and crosscap states are determined. (33 refs).