In a recent paper we discussed, among other things, the effect of duality transformations on the open bosonic string theory. Namely, we conjectured that a macroscopic object, the D-brane, effectively interacts with closed and open string modes in the low energy limit. This conjecture was supported by a string scattering amplitude calculation. In this letter we report on a σ-model calculation in the mixed Dirichlet-Neumann theory. We find an action of the Dirac-Born-Infeld type restricted to (26−k)-dimensions.