Three spectra that have been normalised and had their slopes realigned for comparison. In the middle is the spectrum for one of our sources, ID: 211898 (black line). Top is the spectrum from Danks & Dennefeld (1994) (green line) and bottom is the spectrum from Kacharov et al. (2012) (blue line) that were selected as being the best match for our spectrum by the χ² matching program – both are C-type stars. The vertical lines mark the boundaries of regions 1, 2 and 3 as discussed in the text (Sect. 3.1.1).