Heterotic and Type I string dynamics from eleven dimensions

We propose that the ten-dimensional E8 x E8 heterotic string is related to an eleven-dimensional theory on the orbifold R10 x S1/Z2 in the same way that the Type IIA string in ten dimensions is related to R10 x S1. This in particular determines the strong coupling behavior of the ten-dimensional E8 x E8 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. (orig.)