X-shooter spectra for epochs 3–6 (3.4–6.4 days post-merger) with blackbody fit and emphasis on the P Cygni profiles of Sr⁺ (blue) and an unidentified spectral line at 760 nm (red). For clarity, and to show them on the same plot, the spectra are offset as indicated by the dotted black lines. The feature is clearly noticeable from epoch 3 and onwards. However, the line is poorly constrained in epoch 3 as it less prominent relative to the continuum and overlaps with the most blueshifted parts of the Sr⁺ P Cygni. It is also poorly constrained in epoch 6 and later as the continuum is noisy and less well fit by a blackbody. Epochs 4 and 5 therefore provide the best constraints on the line properties.