Renormalizability of the open string sigma model and emergence of D-branes

Rederiving the one-loop divergences for the most general coupling of the op en string sigma model by the heat kernel technique, we distinguish the clas sical background field from the mean field of the effective action. The lat ter is arbitrary, i.e. does not fulfil the boundary conditions. As a conseq uence a new divergent counter term strongly suggests the introduction of an other external one-form field (beside the usual gauge field), coupled to th e normal derivative at the boundary. Actually such a field has been propose d 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, inc luding the ones for the Born-Infeld action. The most exciting property of t he new coupling is that it enters the coefficient in front of the normal de rivative in Neumann boundary conditions. For certain values of the backgrou nd fields this coefficient vanishes, leading to Dirichlet boundary conditio ns. This provides a natural mechanism for the emergence of D-branes.