Heterotic and type I string dynamics from eleven dimensions

We propose that the ten-dimensional E_8\times E_8 heterotic string is related to an eleven-dimensional theory on the orbifold {\bf R}^{10}\times {\bf S}^1/{\bf Z}_2 in the same way that the Type IIA string in ten dimensions is related to {\bf R}^{10}\times {\bf S}^1. This in particular determines the strong coupling behavior of the ten-dimensional E_8\times E_8 theory. It also leads to a plausible scenario whereby duality between SO(32) heterotic and Type I superstrings follows from the classical symmetries of the eleven-dimensional world, just as the SL(2,{\bf Z}) duality of the ten-dimensional Type IIB theory follows from eleven-dimensional diffeomorphism invariance