Strings from membranes and fivebranes

Under the six-dimensional heterotic/type IIA duality map, a solitonic membrane solution of heterotic string theory transforms into a singular solution of type IIA theory, and should therefore be interpreted as a fundamental membrane in the latter theory. This finding pointed to a gap in the formulation of string theory that was subsequently filled by the discovery of the role of D-branes as the carriers of Ramond-Ramond charge in type II string theory. The roles of compactified eleven-dimensional membranes and fivebranes in five-dimensional string theory are also discussed.