Upper panel: Phase-averaged COS spectrum in F_λ (solid black line, errors in gray). Strong absorption lines are highlighted. A possible Lyα component is in blue, and non-identified metal lines are in red (≈1092, 1108, 1174, 1190, 1193, 1234, 1253, 1277, 1328 Å; see Fig. 8). Narrow carbon and silicon absorption lines (possibly from the interstellar medium or circumstellar material) are highlighted in cyan. In the lower part, the Lyα component is compared to predicted line positions of Lyα and Lyβ as a function of magnetic field, where dashed lines indicate forbidden components (Ruder et al. 1994). Middle left: COS phase-resolved spectrum (five phase bins) over one period, zoomed in on the region with absorption lines. Each phase is shifted vertically by the same amount. Most of the absorption lines do not change significantly with phase, with the exception of the Lyα line, which disappears for half of the period. Middle right: COS light curves in two wavelength ranges: 1100−1200 Å (blue) and 1250−1280 Å (orange). Bottom two panels: Phase resolved COS spectra plotted over two periods. In the upper panel, the color bar indicates the flux in units of 10⁻¹⁵ erg s⁻¹ cm⁻² Å⁻¹. In the lower panel, the phase-resolved spectra have been divided by the phase-averaged spectrum, so the color bar shows the variation with respect to the mean. Two sources of variability can be seen: a strong variation of the continuum between 1100 and 1300 Å, and a variation in the strength of the line at 1344 Å. The absorption is most pronounced when the continuum is at maximum brightness.