the photospheric features. However, the presence of some interstellar absorption is obvious from the line ratio of [[C]{} 1334.5Å]{}/[[C]{} 1335.7Å]{} (Fig. \[f-fit1\] & \[f-fit2\]). Because the latter line originates from a level only 0.008 eV above the real ground state, it is equally populated in a stellar photosphere, but not in the interstellar medium, where the blue component is much stronger in spite of a lower transition probability. Nevertheless, the abundances of C, O, Si, and S are robust, as a sufficient number of excited transitions are present in the photospheric spectrum (Table\[t-idlines\]).

The COS pipeline does not correct for airglow emission. Therefore, the reduced COS spectra can contain geocoronal lines of [[O]{}1302, 1305, 1306Å]{} whose intensity, and, to a lesser extent, profile shape, vary as a function of *HST*’s orbital day/night, and weakly with the Earth-limb angle. Airglow is clearly seen in the spectrum of GALEX1931+0117 (Fig.\[f-fit2\], right panel), which affects the fit to the photospheric [O]{} and [Si]{} lines in this region. For Si, this is a minor problem as there are many additional lines of [Si[-iv]{}]{}. For O, another strong line in the COS spectra is [[O]{} 1152Å]{}.

### Silicon {#s-silicon}

We notice relatively large differences of 