  Orientable open string theories containing both bosons and fermions without
the GSO projection are expected to have the 10 dimensional N=2(A) space-time
supersymmetry in a spontaneously broken phase. We study the low-energy theorem
for the nonlinearly realized N=2 supersymmetry using the effective action for
an unstable D9-brane. It is explicitly confirmed that the 4-fermion open string
amplitudes without the GSO projection obey the low-energy theorem derived from
the nonlinear N=2 supersymmetry. An intimate connection between the existence
of the hidden supersymmetry and the open-open string (s-t) duality is pointed
out.
