Left panel: the model fit to the COS spectrum of PG 1620+260 (with AV = 0.05) illustrates the effect of including the contribution of interstellar Lyα absorption (|$\rm {N (H\, {\small I})}=7.95\times 10^{19}\, \rm {cm}^{-2}$|). Shown as red solid line is the fit including interstellar Lyα, and as blue-dashed line the fit without the interstellar component. The zoomed inset (indicated by the grey box) in the top right corner shows the model fit to the core of Lyα. The best-fitting values, Teff, log g, mass, |$\rm {\chi ^{2}_{r}}$| from both the cases (red: with ISM Lyα, blue: without ISM Lyα) are labelled in the figure. Right panel: the corresponding 95 per cent confidence ellipses (red and grey for two M-R relations) show that the Teff and log g are slightly higher when ISM Lyα is considered in the fit than when it is not (blue-dashed ellipses). The literature values from Liebert et al. (2005, LBH05), G11, GF21, and from Pan-STARRS data (PanST) derived in this work are shown in the figure. Refer to Fig. 1 for a more detailed description of symbols.
