Heterotic and Type I string dynamics from eleven dimensions

We propose that the ten-dimensional E(8) x E(8) heterotic string is related to an eleven-dimensional theory on the orbifoid R(10) x S-1/Z(2) in the same way that the Type IIA string in ten dimensions is related to R(10) x S-1. This in particular determines the strong coupling behavior of the ten-dimensional E(8) x 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,Z) duality of the ten-dimensional type IIB theory follows from eleven-dimensional diffeomorphism invariance.