  The quantum corrections to the counting of statistical entropy for the
5+1-dimensional extremal black string in type-IIB supergravity with two
observers are studied using anomalous Wess-Zumino actions for the corresponding
intersecting D-brane description. The electric-magnetic duality symmetry of the
anomalous theory implies a new symmetry between D-string and D-fivebrane
sources and renders opposite sign for the RR charge of one of the intersecting
D-branes relative to that of the black string. The electric-magnetic symmetric
Hilbert space decomposes into subspaces associated with interior and exterior
regions and it is shown that, for an outside observer, the expectation value of
a horizon area operator agrees with the deviation of the classical horizon area
in going from extremal to near-extremal black strings.
