  We recently proposed a class of type IIB vacua that yield, at low energies,
four--dimensional Minkowski spaces with broken supersymmetry and a constant
string coupling. They are compactifications with an internal five-torus bearing
a five--form flux $\Phi$ and warp factors depending on a single coordinate. The
breaking of supersymmetry occurs when the internal space includes a finite
interval. A probe-brane analysis revealed a gravitational repulsion and a
charge attraction of equal magnitude from the left end of the interval, and a
singularity at the other end. Here we complete the analysis revealing the
presence, at one end, of an effective $O3$ of negative tension and positive
five--form charge. We also determine the values of these quantities, and show
that $T = -\, Q = \Phi$, and characterize the singularity present at the other
end of the interval, which hosts an opposite charge. Finally, we discuss
various forms of the gravity action in the presence of a boundary and identify
a self--adjoint form for its fluctuations.
