limit if no line was identified. Table \[t-idlines\] lists the lines used in this procedure, although not all lines could be used for all four stars. The best-fit models to the COS observations are illustrated in Figs.\[f-fit1\] and \[f-fit2\], and the metal abundances of the four white dwarfs are given in Table\[t-abundances\] (along with the previous abundance studies were carried out for GALEX1931+0117, @vennesetal11-1 [@melisetal11-1]). Notably, upper limits for N were always larger than solar relative to C. For Na, Ti, V, Mn (and additionally Ca in PG0843+516 as well as Ca, Al, P, S, Ni in PG1015+161) the upper limits were larger than solar relative to Si. We have used these (solar) values in the models, but it did not change the atmosphere structure and the results for the detected elements.

### Interstellar line absorption and airglow

In all objects interstellar absorption is visible in the resonance lines of [C]{}, [N]{}, [O]{}, [Si]{}, and [S]{}. In SDSS1228+1040, PG1015+161, and GALEX1931+0117 the interstellar absorption lines are shifted blue-wards with respect to the photospheric lines by velocities of $v = 57$, $36$, and $61$ , respectively. In PG0843+516, $|v| < 7$ , and the interstellar lines are not fully separated from 