Open strings as Z2- orbifolds of closed strings

This paper presents a modular invariant formulation of the open string in terms of the closed string. Chan-Paton factors are understood as multiplicities which arise from a factorization of the closed string. This interpretation of the Chan-Paton factors suggests that the SO(23/2) open string is consistent to all orders of the loop expansion. The authors show that the open string may be viewed as a Z2-orbifold of the closed string. Relations are found between various string theories which seem to reinforce an earlier suggestion by Freund that all string theories are derivable from the D = 26 orientable closed bosonic string