Evolution of fluctuations during graceful exit in string cosmology

We study the evolution of fluctuations in a universe dominated by a scalar field coupled to the Gauss-Bonnet term. During the graceful exit, we found non-negligible enhancements of both curvature perturbation and gravitationa l wave in the long wavelength limit, and we also found a short wavelength i nstability for steep background superinflation just after the completion of the graceful exit. This result for one possible graceful exit mechanism wo uld provide a significant implication on the primordial spectrum from the s tring cosmology. (C) 1999 Elsevier Science B.V. All rights reserved.