D-branes, as I currently understand them, are submanifolds of spacetime on which open strings can end with Dirichlet boundary conditions. On the other hand, type II string theory is a theory of oriented closed strings - the difference between IIA and IIB coming from the choice of GSO projection. Based on these observations, I have the following questions:
- If type II string theory is built from closed strings, why are D-branes considered in the theory?
- If open strings are included in type II, does the GSO projection act on the open strings? If so, how?