Spontaneously broken space-time supersymmetry in open string theory without GSO projection

We argue that orientable open string theory without GSO projection has N=2 space-time supersymmetry in a spontaneously broken phase. The arguments are presented both in Neveu-Schwarz-Ramond and Green-Schwarz formulations. The formal but explicit supersymmetry transformation law for string field is given in the framework of Witten's open string field theory. Our results support a fundamental assumption which lies behind the topological construction of stable D-branes starting from the unstable systems of D9-branes