the residual fixed-pattern noise related to the use of only two FP-POS positions. The resolving power of the COS spectra, as measured from on-orbit data ranges from $\sim15\,000$ at 1150Å to $\sim20\,000$ at 1430Å.

Optical observations {#s-optobs}
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The wavelength spanned by our COS observations does not cover any strong line of either Ca (traditionally the most important tracer of metal pollution in white dwarfs, and an important refractory element) or Mg (one of the major constituents of rocky material in the solar system, including the Earth). Ground-based abundance studies using the [Ca]{} H/K doublet and the [[Mg]{} 4482Å]{} line are already published for GALEX1931+0117 [@vennesetal10-1; @vennesetal11-1; @melisetal11-1]. Two short (10min) VLT/UVES spectra of PG1015+161 were obtained as part of the SPY project [@napiwotzkietal01-1], which @koesteretal05-2 analysed to determine the Ca abundance of PG1015+161 (Sect.\[s-pg1015\]). Here we use the same spectra to determine in addition the abundance of Mg.

We observed PG0843+516 for a total of 2h on the WHT using ISIS with the R600B grating and a $1\arcsec$ slit, covering the Ca and Mg lines at a resolving power of $\simeq2500$ and a S/N of $\approx90$. The data were reduced and calibrated as described in @pyrzasetal12-1.

We also obtained 