We consider the Type IIB string theory in the presence of various extra
7/7ˉ-brane pairs compactified on a warped Calabi-Yau threefold that
admits a conifold singularity. We demonstrate that the volume modulus can be
stabilized perturbatively at a non-supersymmetric AdS4/dS4 vacuum by the
effective potential that includes the stringy (α′)3 correction
obtained by Becker {\it et al.} together with a combination of positive tension
and anomalous negative tension terms generated by the additional
7-brane-antibrane pairs.Comment: 20 pages, 4 figures, parts of introduction and conclusions are
modifie