  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.
