Boundary Conditions and Heterotic Construction in Topological Membrane   Theory
  Using the topological membrane approach to string theory, we suggest a geometric origin for the heterotic string. We show how different membrane boundary conditions lead to different string theories. We discuss the construction of closed oriented strings and superstrings, and demonstrate how the heterotic construction naturally arises from a specific choice of boundary conditions on the left and right boundaries of a cylindrical topological membrane. 
Leith Cooper and Ian I. Kogan (Oxford U.)