  Instanton and seven-brane solutions of type IIB supergravity carrying charges
in the Ramond-Ramond sector are constructed. The singular seven-brane has a
quantized \RR \ \lq magnetic' charge whereas its dual is the instanton, which
is non-singular in the string frame and has an associated {\it global} \lq
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.
