AMBER data, best-fit model, and pure MARCS model fit for KW Sgr. Upper left: Normalised flux for the RSG KW Sgr (grey), as observed with VLT/AMBER for the K − 2.1 μm bands. Our best-fit model is shown in red, and the pure MARCS model fit is shown in green. As expected, the fluxes are well represented by both our fit and MARCS. Upper right: Same as the upper-left panel, but for the K − 2.3 μm band. Lower left: Same as the upper-left panel, but for the |V|² with the different baselines. Lower right: Same as the lower-right panel, but for the K − 2.3 μm band and different baselines. Our model can represent the data better than a pure MARCS model.