Visibility in the N band plotted against the wavelength. The black curve represents the weighted mean of the observations. U Aql is not presented since we only measured the correlated flux and the X Sgr photometry in the N band was not usable. The spectral band between 9.3 and 10 micron was removed because the atmosphere is not transmissive. In the case of T Mon, we overplot the weighted mean of MIDI/VLTI observation as given by Gallenne et al. (2013) for comparison.