Left: Infrared light curves (K_s-, W1-, and W2-band) of VVV-WIT-13. The W1-band error bars (orange) are the standard deviation of the magnitude spread in each epoch before binning. The typical error bar in the K_s-band (< 0.1 mag) is smaller than the symbol size. A sinusoidal fitting result is presented by the grey line. The spectroscopic epochs are marked by the vertical dashed lines with the same colour scheme as in the right panels. Right: Near-infrared spectra of VVV-WIT-13 obtained in 2021, 2023, and 2024. The key spectral features are labelled on the plot.