The mirror transform of type I vacua in six dimensions

We study certain compactifications of the type I string on K3. The three topologically distinct choices of gauge bundles for the type I theory are shown to be equivalent to type IIB orientifolds with different choices of background anti-symmetric tensor field flux. Using a mirror transformation we relate these models to orientifolds with fixed seven planes, and without any anti-symmetric tensor field flux. This map allows us to relate these type I vacua to particular six-dimensional F-theory and heterotic string compactifications.