Illustrating, in the case of HD 85567, the degeneracy that arises when fitting a simple model (star+circumstellar) to interferometric visibility data. Top panel: radius a and flux fraction f_c (in the MCMC chain) of the circumstellar component, crosses labeled A and B mark the 10% and 90% points in the respective marginal distributions. Bottom panel: observed data for V² (middle spectral channel only), and visibility profiles for (isotropic) Gaussians with the parameters at points A and B respectively.