Reduced χ² map as a function of the flux contribution f_CSE and FWHM (R_CSE) of the Gaussian CSE model in the L and M bands: U Aql, 7.02 d (a), η Aql, 7.17 d (b), β Dor, 9.84 d (c), ζ Gem, 10.15 d (d), TT Aql, 13.75 d (e), T Mon, 27.03 d (f), and U Car, 38.87 d (g). The dashed cyan, orange, and purple lines represent different CSE radii (FWHM) for which we provide the upper limit on the CSE flux contribution. It is displayed as a square (see Table 5). The weak flux contribution (<5%) does not permit us to constrain the CSE dimensions. Conversely, the flux of the compact envelope (<3–4 R_⋆) cannot be determined. Large and bright models (R>10 R_⋆f > 10%, purple squares) can be excluded, however.