Spontaneously broken space–time supersymmetry in open string theory without GSO projection Original Research Article

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 Wittenʹs 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.