A note on orientifolds and dualities of type 0B string theory

We generalize the construction of four dimensional non-tachyonic orientifol ds of type 0B string theory to non-supersymmetric backgrounds. We construct a four dimensional model containing self-dual D3 and D9-branes and leading to a chiral anomaly-free massless spectrum. Moreover, we discuss a further tachyon-free six dimensional model with only D5 branes. Eventually, we spe culate about strong coupling dual models of the ten-dimensional orientifold s of type 0B. (C) 1999 Published by Elsevier Science B.V. All rights reserv ed.