($17\,000\,\mathrm{K}<{\mbox{$T_{\mathrm{eff}}$}}<25\,000$K) DA white dwarfs were drawn from the compilations of @liebertetal05-1 and @koesteretal09-2, supplemented with a few recent discoveries [e.g. @gaensickeetal06-3; @vennesetal10-1]. Our sample also includes a small number of post-common envelope binaries (PCEBs) in which the white dwarf accretes from the wind of the M-dwarf companion. These systems were selected from @schreiber+gaensicke03-1 and @farihietal10-3 with the same cut on white dwarf temperature and cooling age. Under the assumption that the M-dwarfs have a solar-like composition, the white dwarfs in PCEBs serve as “abundance standards” for our abundances analyses and diffusion calculations.

*HST*/COS spectroscopy {#s-hstobs}
----------------------

PG0843+516, PG1015+161, and GALEXJ193156.8+011745 (henceforth GALEX1931+0117) were observed as part of our snapshot survey, with exposure times of 1420s, 1424s, and 800s, respectively. We used the G130M grating with a central wavelength of 1291Å, which covers the wavelength range $1130-1435$Å, with a gap at $1278-1288$Å due to the space between the two detector segments. To mitigate the fixed pattern noise that is affecting the COS far-ultraviolet detector, we split the exposure time equally between two FP-POS positions (1&4, the limited duration of the snapshot visits did not allow to use the full set of four different FP-POS positions).

We also report COS observations of 