  It is well-known that the SO(32) and $E_8\times E_8$ heterotic strings can be
continuously connected to each other in nine dimensions. Since the
strong-coupling duals of these theories are respectively the SO(32) Type I
theory and M-theory compactified on a line segment, there should be a
corresponding continuous connection between the Type I string and M-theory. In
this paper, we give an explicit construction of this dual connecting theory.
Our construction also enables us to realize the $E_8\times E_8$ heterotic
string as a D-string soliton of the Type I theory. This provides a useful
alternative description of the D-brane bound states previously discussed from a
Type I' point of view.
