  We compute the amplitude for the radiation of massless NS-NS closed string
states from the interaction of two moving D-branes. We consider particle-like
D-branes with reference to 4-dimensional spacetime, in toroidal and orbifold
compactifications, and we work out the relevant world sheet propagators within
the moving boundary state formalism. We find no on-shell axion emission. For
large inter-brane separation, we compute the spacetime graviton emission
amplitude and estimate the average energy radiated, whereas the spacetime
dilaton amplitude is found to vanish in this limit. The possibility of emission
of other massless states depends on the nature of the branes and of the
compactification scheme.
