  F-theory on appropriately fibered Spin(7) holonomy manifolds is defined to
arise as the dual of M-theory on the same space in the limit of a shrinking
fiber. A class of Spin(7) orbifolds can be constructed as quotients of
elliptically fibered Calabi-Yau fourfolds by an anti-holomorphic involution.
The F-theory dual then exhibits one macroscopic dimension that has the topology
of an interval. In this work we study the weak-coupling limit of a subclass of
such constructions and identify the objects that arise in this limit. On the
Type IIB side we find space-time filling O7-planes as well as O5-planes and
orbifold five-planes with a (-1)^{F_L} factor localised on the interval
boundaries. These orbifold planes are referred to as X5-planes and are S-dual
to a D5-O5 system. For other involutions exotic O3-planes and X3-planes on top
of a six-dimensional orbifold singularity can appear. We show that the objects
present preserve a mutual supersymmetry of four supercharges in the bulk of the
interval and two supercharges on the boundary. It follows that in the
infinite-interval and weak-coupling limit full four-dimensional N=1
supersymmetry is restored, which on the Type IIA side corresponds to an
enhancement of supersymmetry by winding modes in the vanishing interval limit.

High Energy Physics - Theory