  We study some aspects of the strong coupling dynamics of Dirichlet six
branes, anti- six branes, and orientifold planes by using the equivalence of
type IIA string theory and M-theory on S^1. In the strong coupling limit there
exists static configuration of brane and anti-brane at arbitrary separation,
suspended in an external magnetic field. The mass of the open string stretched
between the brane and the anti-brane approaches a finite positive value even
when the branes coincide. Similar result is obtained for a Dirichlet six brane
on top of an orientifold six plane. We also derive the anomalous gravitational
interaction on the brane and the orientifold plane from M-theory.
