A non-supersymmetric ten-dimensional open-string theory is constructed
as an orbifold of type I string theory, and as an orientifold of the
bosonic type B theory. It is purely bosonic, and cancellation of massl
ess tadpoles requires the gauge group to be SO(32) x SO(32). The spect
rum of the theory contains a closed-string tachyon, and open-string ta
chyons in the (32,32) multiplet. The D-branes of this theory are analy
zed, and it is found that the massless excitations of one of the 1-bra
nes coincide with the world-sheet degrees of freedom of the D = 26 bos
onic string theory compactified on the SO(32) lattice, This suggests t
hat the two theories are related by S-duality. (C) 1997 Elsevier Scien
ce B.V.