2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/159818We 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.30 pages, 4 figures; v2: citations addedHigh Energy Physics - Theorytext