  Integrable defects in two-dimensional integrable models are purely
transmitting thus topological. By fusing them to integrable boundaries new
integrable boundary conditions can be generated, and, from the comparison of
the two solved boundary theories, explicit solutions of defect models can be
extracted. This idea is used to determine the transmission factors and defect
energies of topological defects in sinh-Gordon and Lee-Yang models. The
transmission factors are checked in Lagrangian perturbation theory in the
sinh-Gordon case, while the defect energies are checked against defect
thermodynamic Bethe ansatz equations derived to describe the ground-state
energy of diagonal defect systems on a cylinder. Defect bootstrap equations are
also analyzed and are closed by determining the spectrum of defect bound-states
in the Lee-Yang model.
