S-Duality as an Open String Gauge Symmetry

The paper discusses the connection between S-duality and string-theoretic picture-changing formalism. The singular limit of the picture-changing transformation at zero momentum leads to the presence of the topological 5-form charge in the type IIB superalgebra which is attributed to the M5-brane. The topological charge defines the boundary D-brane state, which also is the analogue of the monopole part of the Olive-Witten's result in field theory.The correlation functions involving this non-perturbative state are computed.The non-perturbative D-brane states are associated with the ghost number cohomologies, introduced in the paper. Connections with M(atrix) theory are discussed.