Consistency conditions for orientifolds and D-manifolds

We study superstrings with orientifold projections and with generalized open string boundary conditions (D branes). We find two types of consistency condition, one related to the algebra of Chan-Paton factors and the other to cancellation of divergences. One consequence is that the Dirichlet five branes of the type I theory carry a symplectic gauge group, as required by string duality. As another application we study the type I theory on a $K3$ Z₂ orbifold, finding a family of consistent theories with various unitary and symplectic subgroups of U(16)×{}U(16). We argue that the $K3$ orbifold with spin connection embedded in gauge connection corresponds to an interacting conformal field theory in the type I theory.