Orbifolds of M-theory and string theory

It is shown that many of the conjectured dualities involving orbifold compactification of M-theory follow from the known dualities involving M-theory and string theory in ten dimensions, and the ansatz that orbifolding procedure commutes with the duality transformation. This ansatz also leads to a new duality conjecture, namely that M-theory compactified on T^8/Z_2 is dual to type I string theory on T^7. In this case the `twisted sector states' in M-theory live on sixteen membranes transverse to the internal manifold