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 10 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 16 membranes transverse to the internal manifold.