  We develop the moduli-space approximation for the low energy regime of
BPS-branes with a bulk scalar field to obtain an effective four-dimensional
action describing the system. An arbitrary BPS potential is used and account is
taken of the presence of matter in the branes and small supersymmetry breaking
terms. The resulting effective theory is a bi-scalar tensor theory of gravity.
In this theory, the scalar degrees of freedom can be stabilized naturally
without the introduction of additional mechanisms other than the appropriate
BPS potential. We place observational constraints on the shape of the potential
and the global configuration of branes.
