  We study the asymptotic limits of the heterotic string theories compactified
on tori. We find a bilinear form uniquely determined by dualities which becomes
Lorentzian in the case of one spacetime dimension. For the case of the SO(32)
theory, the limiting descriptions include SO(32) heterotic strings, 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.
