Bulk induced boundary perturbations

The influence of closed string moduli on the D-brane moduli space is studied from a worldsheet point of view. Whenever a D-brane cannot be adjusted to an infinitesimal change of the closed string background, the corresponding exactly marginal bulk operator ceases to be exactly marginal in the presence of the brane. The bulk perturbation then induces a renormalisation group flow on the boundary whose end-point describes a conformal D-brane of the perturbed theory. We derive the relevant RG equations in general and illustrate the phenomenon with a number of examples, in particular the radius deformation of a free boson on a circle. At the self-dual radius we can give closed formulae for the induced boundary flows which are exact in the boundary coupling constants.