Top left: observed X-shooter spectrum of GJ551 (green line) (Pavlenko et al. 2017) and flux ratio F₄₈ ∕F_all (red line); the green line shows the observed X-shooter fluxes in the Proxima spectrum. x₁, ...x₅ mark our spectral ranges of interest. Top right: band heads of TiO isotopologues computed for a 2900/4.5/0.0 model atmosphere. All observed spectra are shifted to the zero radial velocity frame. Middle panels: theoretical spectra of bands of TiO isotopologues computed across the x₂ it (right panel) and x₃ (left panel) spectral ranges. Upward arrows show the positions of head bands of the TiO isotopologues. Downward arrows label two Fe I lines seen in absorption. Bottom panels: theoretical spectra of bands of TiO isotopologues computed across x₄ (right panel)and x₅ (left panel) spectral ranges. The positions of the molecular band heads are shown by arrows. In all panels the observed spectra of Acturus and Proxima Cen (GJ551) are shown by thin and thick red lines, respectively, to demonstrate the locations of notable atomic and molecular features in the observed spectra.