On supersymmetric interfaces for string theory

We construct the world-sheet interface which preserves space-time supersymmetry in type II superstring theories in the Green-Schwarz formalism. This is an analog of the conformal interface in two-dimensional conformal field theory. We show that a class of the supersymmetric interfaces generates T-dualities of type II theories, and that these interfaces have a geometrical interpretation in the doubled target space. We compute the partition function with a pair of the supersymmetric interfaces inserted, from which we read off the spectrum of the modes coupled to the interfaces and the Casimir energy between them. We also derive the transformation rules under which a set of D-branes is transformed to another by the interface.