  We consider, in a string theory framework, physical processes of
phenomenological interest in models with a low string scale. The amplitudes we
study involve tree-level virtual gravitational exchange, divergent in a
field-theoretical treatment, and massive gravitons emission, which are the main
signatures of this class of models. First, we discuss the regularization of
summations appearing in virtual gravitational (closed string) Kaluza-Klein
exchanges in Type I strings. We argue that a convenient manifestly ultraviolet
convergent low energy limit of type I string theory is given by an effective
field theory with an arbitrary cutoff $\Lambda$ in the closed (gravitational)
channel and a related cutoff $M_s^2/\Lambda$ in the open (Yang-Mills) channel.
We find the leading string corrections to the field theory results. Second, we
calculate exactly string tree-level three and four-point amplitudes with gauge
bosons and one massive graviton and examine string deviations from the
field-theory result.
