Spectra of the sample of objects studied here. Spectra are normalised to unity at 2.6 µm. Objects are coloured and overlap according to their spectral types. Double-dip objects are shown at the top in orange and cliff objects are at the bottom in red. The thick lines represent the average of each group. Huya and 2004 PG₁₁₅ are presented in the middle in grey to highlight their mixed spectral characteristics. Except for the cliff group, the spectra are offset vertically for clarity. The most characteristic bands of CO₂, CO, and CH₃OH are marked with vertical lines. The variability of CH₃OH bands within the cliff group was studied by Brunetto et al. (2025).