Optical depth of the different sources obtained from the SED fitting in Sect. 3 as a function of the luminosity of the stars (left panel) and of the distance of the internal border of the dusty region from the centre of the star (right). Red squares and blue triangles indicate stars surrounded by carbonaceous dust and silicates, respectively. Crossed red squares refer to stars exhibiting evidence of carbon dust, despite their luminosities being consistent with an M-type nature.