Prescriptions for Off-Shell Bosonic String Amplitudes
  We give, in the framework of the bosonic string theory, simple prescriptions for computing, at tree and one-loop levels, off-shell string amplitudes for open and closed string massless states. In particular we obtain a tree amplitude for three open strings that in the field theory limit coincides with the three-gluon vertex in the usual covariant gauge and two-string one-loop amplitudes satisfying the property of transversality. 
L. Cappiello, A. Liccardo, R. Pettorino, F. Pezzella (Dipartimento di   Scienze Fisiche, Universita' di Napoli and I.N.F.N.,...