A note on orientifolds and F-theory

An orientifold of Type-IIB theory on a K3 realized as a Z2 orbifold is constructed which corresponds to F-theory compactification on a Calabi-Yau orbifold with Hodge numbers (51, 3). The T-dual of this model is analogous to an orbifold with discrete torsion in that the action of orientation reversal has an additional phase on the twisted sectors, and both 9-branes and 5-branes carry orthogonal gauge groups. An orientifold of the Z3 orbifold and its relation to F-theory is briefly discussed.