Factorization of residues of off-shell string amplitudes

All normalization factors are supplied for the calculation of off-shell string amplitudes in the formalism of Cohen et al. Wave function normalization constants are determined from the residues of poles in the two-point function. For amplitudes with one or two strings off-shell and an arbitrary number of on-shell particles, the residue of a pole in an off-shell channel factorizes into the product of the square root of the wave function normalization constant and the correctly normalized amplitude with the particle in the relevant channel on-shell. Factorization requires the same relation between the coupling constant defined by the topological term in the action and the coupling constant appearing in vertex operators that was derived previously from unitarity of S-matrix elements. (orig.)