Variability of the flux (calibrated using the spectral transfer function) for all our observations. The spectra were divided by mean flux in the continuum region (2.22–2.28 μm). Upper panel: R Car. The spectra were vertically shifted, ordered, and color-coded by phase φ_vis (φ_vis = 0 corresponds to brightness maximum). Lower panel: same, but for VX Sgr. The spectra were vertically shifted, ordered by observation date, and color-coded by the nearest m_vis values from AAVSO.