Open String Diagrams I: Topological Type
  An arbitrary Feynman graph for string field theory interactions is analysed and the homeomorphism type of the corresponding world sheet surface is completely determined even in the non-orientable cases. Algorithms are found to mechanically compute the topological characteristics of the resulting surface from the structure of the signed oriented graph. Whitney's permutation-theoretic coding of graphs is utilized. 
Subhashis Nag and Parameswaran Sankaran (The Institute of Mathematical   Sciences C.I.T.Campus Madras)