Comparison between the mid-IR synthetic spectra and the MIDI calibrated spectra and between the near-IR photometry and K-band synthetic spectrum for S Ser. The spectra were not fitted but used as constraints to model the visibilities.The synthetic spectra for the N band are based on the three-layer model, one per range of wavelengths: 7.80−9.30 μm, 9.37−11.50 μm, and 11.55−13.26 μm. The light gray spectrum model for S Ser has been obtained fitting the visibilities with a single envelope.