  We consider a supersymmetric system of D-5-branes compactified on a 5-torus
with a self-dual background field strength on a 4-torus and carrying
left-moving momentum along a circle. The corresponding supergravity solution
describes a 5-dimensional black hole with a regular horizon. The entropy of
this black hole may be explained in terms of the Landau degeneracy for open
strings stretching between different branes. In the gauge theory approximation
this D-5-brane system is described by a super Yang-Mills theory with a t'Hooft
twist. By choosing a supersymmetric branch of the theory we obtain perfect
agreement with the entropy formula. The result relies on the number of massless
torons associated with the gauge field components that obey twisted boundary
conditions.
