  We consider tachyon condensation in unstable ten-dimensional heterotic string
theory with gauge group E8. In the background of a lightlike linear dilaton
rolling to weak coupling, we find an exact solution in which the theory decays
to a stable ground state. The final state represents a new, modular-invariant
perturbative string theory, tachyon-free in nine spacetime dimensions with a
spacelike dilaton gradient, E8 gauge group and no spacetime supersymmetry.
