Heterotic and Type I string dynamics from eleven dimensions

We propose that the ten-dimensional E 8 × E 8 heterotic string is related to an eleven-dimensional theory on the orbifold R 10 × S1/Z 2 in the same way that the Type IIA string in ten dimensions is related to R 10 × S1 . This in particular determines the strong coupling behavior of the ten-dimensional E 8 × 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.