Extinction in I Zw 1 derived from the COS and OM 2015 data. The extinction curve (red line) is based on the Gordon et al. (2003) SMC bar average curve (black). The relatively smaller amount of reddening above 8 eV is described by an additional parameter controlling the far-UV curvature, constrained from the fit to c₄ = −0.34 ± 0.05 (see the text for more details). This 1σ uncertainty is visualised with the shaded area surrounding the curve. The Fitzpatrick (1999) Galactic extinction curve (grey) is given for reference. The positions of the data points used for the fitting are depicted with grey dots on the top.