COS spectrum of RX J0122.9–7521 (in grey) with our best-fit photospheric model overlaid in red (175 000 K and log g = 7.7; other parameters are listed in Table 2). The most prominent photospheric lines are highlighted in red, and interstellar lines are marked in blue. In black, the forbidden C IV 3d–4d transitions at 1170.13 Å and 1170.33 Å are highlighted. To match the observed spectra, the absorption feature of H I Lyman-alpha was additionally modelled with a column density of n_HI = 4.5 × 10²⁰ cm⁻².