  Rederiving the one-loop divergences for the most general coupling of the open
string sigma model by the heat kernel technique, we distinguish the classical
background field from the mean field of the effective action. The latter is
arbitrary, i.e. does not fulfil the boundary conditions. As a consequence a new
divergent counter term strongly suggests the introduction of another external
one-form field (beside the usual gauge field), coupled to the normal derivative
at the boundary. Actually such a field has been proposed in the literature for
different reasons, but its full impact never seems to have thoroughly
investigated before. The beta function for the resulting renormalizable model
is calculated and the consequences are discussed, including the ones for the
Born-Infeld action. The most exciting property of the new coupling is that it
enters the coefficient in front of the normal derivative in Neumann boundary
conditions. For certain values of the background fields this coefficient
vanishes, leading to Dirichlet boundary conditions. This provides a natural
mechanism for the emergence of D-branes.
