temperatures derived from the ultraviolet data when compared to the values based on optical spectroscopy. A similar trend is seen for DA white dwarfs with ${\mbox{$T_{\mathrm{eff}}$}}\sim20\,000$K in @lajoie+bergeron07-1, who compared the effective temperatures derived from optical and (*International Ultraviolet Explorer *) ultraviolet spectroscopy. We carried out a range of test calculations to explore the effect of these systematic uncertainties in ${\mbox{$T_{\mathrm{eff}}$}}$ and $\log g$ on the derived metal abundances (Sect. \[s-abundances\]). The abundances and mass fluxes do not change by more than $\simeq0.1$dex, which is less than the typical uncertainty of our fits, and the abundance ratios vary by much less. Hence, the discussion in Sect. \[s-debrisnature\] and \[s-mdot\] is not affected by the systematic uncertainties in ${\mbox{$T_{\mathrm{eff}}$}}$ and $\log g$.

Finally, to assess the possible effect that the presence of metals has on the effective temperature and surface gravity, we computed a small grid of models for the two most metal-polluted stars (PG0843+516, GALEX1931+0117), including metals at the abundances determined in Sect.\[s-abundances\], and re-fitted the *HST*/COS spectra. For both stars, the best-fit ${\mbox{$T_{\mathrm{eff}}$}}$ and $\log g$ did not change significantly, and we therefore adopted the atmospheric parameters from the pure-hydrogen fits for all four targets.

  Ion              Vacuum wavelengths \[Å\]
  