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 $K3$ ${Z}_{2}$ orbifold, finding a family of consistent theories with various unitary and symplectic subgroups of U(16)\ifmmode\times\else\texttimes\fi{}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.