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 gravitational
wave in the long wavelength limit, and we also found a short wavelength
instability for steep background superinflation just after the completion of
the graceful exit. This result for one possible graceful exit mechanism would
provide a significant implication on the primordial spectrum from the string
cos mology.Comment: 5 pages, 5 figures, Revtex, to appear in Phys. Lett.
