The collision of plane waves corresponding to massless states of closed str
ing is considered in D-dimensional space-time. The reduced tree level effec
tive action is known to be manifestly O (d, d) invariant, d being the numbe
r of transverse spatial dimensions in the collision process. We adopt a cos
et space reformulation of the effective two-dimensional theory and discuss
the relation of this process with classical integrable systems in two dimen
sions in the presence of gravity. We show how it is possible to generate ne
w backgrounds for the scattering process, from known background solutions t
o the equations of motion, in the coset reformulation. We present explicit
calculations for the case of four space-time dimensions as an illustrative
example. (C) 2001 Elsevier Science BN. All rights reserved.