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tokens: (by ∼400 km s −1 ) than in Cano et al. ( 2018 ). Figure 9. Time evolution of the pEWs for features 1, 3, 4, 7, and 8 together with the Si ii expansion velocity evolution (bottom right panel). Black circles are the measured pEWs after lens- and host galaxy subtraction and de-reddening. Transparent points and shaded bands are individual measurements and the binned mean (±1σ) of low-redshift SNe from Folatelli et al. ( 2013 ), for all (red) and ‘Normal’ (blue) SNe Ia in their sample. The black and red dashed lines shows the pEW and velocity evolution of the Hsiao template and SN 2007le, respectively. Turning to the pEW measurements, we do not see any significant deviations from the time-evolution of SNe Ia in Folatelli et al. ( 2013 ). In Fig. 9 , the black points show the pEW measurements of iPTF16geu compared to all (red points) or ‘normal’ (blue points and blue shaded region) SNe Ia in Folatelli et al. ( 2013 ). However, we note that some measurements are outside the 1σ range, e.g. late-time pEW1 (likely due to improper lens and host galaxy removal), pEW3 and pEW7 (where the telluric corrections are imperfect). 5 TIME-DELAY MEASUREMENTS FROM SPECTRA OF SPATIALLY RESOLVED LENS IMAGES In our single epoch of HST spectroscopy, we can spatially resolve two spectra: one spectrum corresponding to Image 2 and one spectrum corresponding to Images 1, 3, and 4. However, Images 3 and 4 are subdominant, contributing with 7 per cent and 5 per cent to the total flux in F 814 W at this epoch, respectively. Hence, we will treat this spectrum as stemming from Image 1. By comparing the time evolution of spectral features between the spectra, we can in principle provide an independent measurement of the time delays between the SN images. However, in our case we are limited by the coarse ( R ∼ 210) spectral resolution. To complicate things further, at the epoch (+29 d) the Si ii absorption feature is no longer well-defined, displaying two or more local minima. Age dating of SN spectra is routinely done using cross-correlation techniques (e.g. snid ; Blondin Tonry 2007 ), and have been successful in measuring cosmic time-dilation (Blondin et al. 2008 ). We construct a simple χ 2 -statistic, fitting Hsiao template spectra (Hsiao et al. 2007 ) at different phases to the spectra of the spatially resolved SN images. This allows us to simultaneously fit for residual lens and host galaxy contamination using our template spectrum, and subtract a fraction of that flux (0.80 ± 0.03 and 0.19 ± 0.02 for Images 1 and 2, respectively) from the resolved SN spectra, so that the subtracted SN spectra and Hsiao templates at each phase match the host subtracted F 814 W photometry (see upper panel of Fig. 10 ). Figure 10. Top panel: HST spectra of the spatially resolved Images 1 + 3 + 4 (blue line) and Image 2 (green line). The summed spectra (red line) show very good agreement with the ground-based Keck spectrum (black line), observed at the same time. Bottom panel: χ 2 minima of