dwarf radii from ${\mbox{$T_{\mathrm{eff}}$}}$ and $\log g$. The mass fluxes (=accretion rates) of the individual elements, as well as their sum, are shown in Fig.\[f-mdot\] and discussed in Sect. \[s-mdot\]. The number abundances of the circumstellar debris are then calculated from the diffusion fluxes via $$\mathrm{\frac{N(X)}{N(Si)}=\frac{\dot M(X)}{\dot M(Si)}\frac{A(Si)}{A(X)}}$$ where A is the atomic mass. The implications that these abundances have on the nature and origin of the circumstellar debris are discussed in detail in Sect.\[s-debrisnature\].

![\[f-mdot\] Accretion rates of the elements detected in our four targets. Their sum is given in the right-most column.](mdot.ps){width="\columnwidth"}

Notes on individual white dwarfs {#s-notes}
================================

In the following sections, we give a brief overview of previous work on the four white dwarfs that we have analysed, as well as a summary of the key results of our observations.

PG0843+516
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PG0843+516 was identified as a DA white dwarf in the Palomar-Green Survey [@greenetal86-1], and @liebertetal05-1 obtained ${\mbox{$T_{\mathrm{eff}}$}}=23\,870\pm392$K, $\log g=7.90\pm0.05$ from the analysis of a high-quality optical spectrum. The best fit to our *HST* data was ${\mbox{$T_{\mathrm{eff}}$}}=23\,095 \pm 230$K, $\log g=8.17\pm0.06$. Our COS spectrum reveals PG0843+516 to be an extremely polluted DAZ white dwarf (Fig.\[f-cos\] & \[f-fit1\]), with an accretion rate of $\simeq10^9\,\mathrm{g\,s^{-1}}$, placing it 