We study the asymptotic limits of the heterotic string theories compactifie
d on tori. We find a bilinear form uniquely determined by dualities which b
ecomes Lorentzian in the case of one spacetime dimension. For the case of t
he SO(32) theory, the limiting descriptions include SO(32) heterotic string
s, type I, type IA and other T-duals, M-theory on K3, type IIA theory on K3
and type IIB theory on K3 and possibly new limits not understood yet.