  We derive the Bekenstein-Hawking entropy formula for four-dimensional
Reissner-Nordstrom extremal black holes in type II string theory. The
derivation is performed in two separate (T-dual) weak coupling pictures. One
uses a type IIB bound state problem of D5- and D1-branes, while the other uses
a bound state problem of D0- and D4-branes with macroscopic fundamental type
IIA strings. In both cases, the D-brane systems are also bound to a
Kaluza-Klein monopole, which then yields the four-dimensional black hole at
strong coupling.
