  We discuss the effect of the dynamics of D-branes on moduli stabilization in
type IIB string theory compactifications, with reference to a concrete toy
model of T6/Z3 orientifold compactification with fractional D3-branes and
anti-D3-branes at orbifold fixed points. The resulting attractive forces
between anti-D3-branes and D3-branes, together with the repulsive forces
between anti-D3-branes and O3-planes, can affect the stability of the compact
space. There are no complex structure moduli in T6/Z3 orientifold, which should
thus capture some generic features of more general settings where all complex
structure moduli are stabilized by three-form fluxes. The simultaneous presence
of branes and anti-branes brings along the breaking of supersymmetry. Non-BPS
combinations of this type are typical of "brane supersymmetry breaking", and
are a necessary ingredient in the KKLT scenario for stabilizing the remaining
Kahler moduli. The conclusion of our analysis is that, while mutual D-brane
interactions sometimes help Kahler moduli stabilization, this is not always the
case.
