  We study a family of unstable heterotic string theories in more than ten
dimensions which are connected via tachyon condensation to the ten-dimensional
supersymmetric vacuum of heterotic string theory with gauge group SO(32).
Calculating the spectrum of these theories, we find evidence for an S-duality
which relates type I string theory in ten dimensions with n additional
ninebrane-antininebrane pairs to heterotic string theory in 10+n dimensions
with gauge group SO(32+n). The Kaluza-Klein modes of the supercritical
dimensions are dual to non-singlet bound states of open strings.
