Upper left: Near-infrared spectrum of VVV-WIT-13 taken during the brightness plateau. The best-fit molecular spectra (AlO, H₂O, and CO) are presented in the lower left panel; the effective temperature and column density are labelled in the plot. The coloured squares mark the spectral regions presented in the right panels. The spectral regions among J, H, and K bandpasses are shown in grey, which are dominated by water absorption features. Right: Zoomed-in views of two regions with molecular absorption features, with radial velocity listed in the heliocentric frame. The theoretical wavelengths of ¹³CO absorption bandheads are shown as dashed grey lines.