Is string theory a theory of strings

Abstract Recently a great deal of evidence has been found indicating that type IIA string theory compactified on K 3 is equivalent to heterotic string theory compactified on T 4 . Under the transformation which relates the two theories, the roles of fundamental and solitonic string solutions are interchanged. We show that there exists a solitonic membrane solution of the heterotic string theory which becomes a singular solution of the type IIA theory, and should therefore be interpreted as a fundamental membrane in the latter theory. We speculate upon the implications that the complete type IIA theory is a theory of membranes, as well as strings.