Instantons and seven-branes in type IIB superstring theory

Instanton and seven-brane solutions of type IIIB supergravity carrying charges in the Ramond-Ramond sector are constructed. The singular seven-brane has a quantized R x R 'magnetic' charge whereas its dual is the instanton, which is non-singular in the string frame and has an associated global 'electric' charge. The product of these charges is constrained by a Dirac quantization condition. The instanton has the form of a space-time wormhole in the string frame, and is responsible for the non-conservation of the Noether current.