Relevant boundary perturbations of CFT: A case study

We consider simple CFT models which contain massless bosons, or massless fermions or a supersymmetric combination of the two, on the strip. We study the deformations of these models by relevant boundary operators. In particular, we work out the details for a boundary operator with a quadratic dependence on the fields and argue that some of our results can be extended to a more general situation. In the fermionic models, several subtleties arise due to the doubling of zero modes at the UV fixed point and a ``GSO projected'' RG flow. We attempt to resolve these issues and to discuss how bulk symmetries are realised along the flow. We end with some speculations on possible string theory applications of these results.