ENTROPY AND TEMPERATURE OF BLACK 3-BRANES

We consider slightly nonextremal black 3-branes of type IIB supergravi ty and show that their Bekenstein-Hawking entropy agrees, up to a myst erious factor; with an entropy derived by counting non-BPS excitations of the Dirichlet 3-brane. These excitations are described in terms of the statistical mechanics of a (3+1)-dimensional gas of massless open string states. This is essentially the classic problem of blackbody r adiation. The blackbody temperature is related to the temperature of t he Hawking radiation. We also construct a solution of type IIB supergr avity describing a 3-brane with a finite density of longitudinal momen tum. For extremal momentum-carrying 3-branes the horizon area vanishes . This is in agreement with the fact that the BPS entropy of the momen tum-carrying Dirichlet 3-branes is not an extensive quantity.