HST spectrum of AT2017gfo at 4.9 days post-merger from Tanvir et al. (2017). The grey shaded region indicates the continuum constraints from the interpolation of the X-shooter spectra taken at 4.4 and 5.4 days post-merger. Regardless of the exact continuum modelling, a P Cygni absorption/emission feature around 1400 nm is clearly visible. Domoto et al. (2022) suggested that this feature could be due to La III lines (here indicated with dashed blue lines). The characteristic velocity of this feature is relatively low, v_ph = 0.07 ± 0.01c, but may be systematically biased by the strong nearby features (including the prominent reverberation emission peak around 1.1 μm).