Nonlinear supersymmetry without the GSO projection and unstable D9-brane

Orientable open string theories containing both bosons and fermions without the GSO projection are expected to have the 10-dimensional N = 2 space-tim e supersymmetry in a spontaneously broken phase. We study the low-energy th eorem for the nonlinearly realized N = 2 supersymmetry using the effective action for an unstable D9-brane, It is explicitly confirmed that the 4-ferm ion open string amplitudes without the GSO projection obey the low-energy t heorem derived from the nonlinear N = 2 supersymmetry. An intimate connecti on between the existence of the hidden supersymmetry and the open-open stri ng (s-t) duality is pointed out. (C) 2001 Elsevier Science B.V. All rights reserved.