On least action D-branes

We discuss the effect of relevant boundary terms on the nature of branes. T his is done for toroidal and orbifold compactifications of the bosonic stri ng, Using the relative minimalization of the boundary entropy as a guiding principle, we uncover the more stable boundary conditions at different regi ons of moduli space. In some cases, Neumann boundary conditions dominate fo r small radii while Dirichlet boundary conditions dominate for large radii. The c = 1 and c = 2 moduli spaces are studied in some detail. The antisymm etric background field B is found to have a more limited role in the case o f Dirichlet boundary conditions. This is due to some topological considerat ions. The results are subjected to T-duality tests and the special role of the points in moduli space fixed under T-duality is explained from least-ac tion considerations, (C) 1999 Elsevier Science B.V.