around different white dwarfs is remarkably similar. This is analogous to the nearly constant abundance ratio of these two refractory lithophile elements found among most bodies in the solar system.

    Based on the detection of all major elements of the circumstellar debris, we calculate accretion rates of $\simeq1.7\times10^8\,\mathrm{g\,s^{-1}}$ to $\simeq1.5\times10^9\,\mathrm{g\,s^{-1}}$. Finally, we detect additional circumstellar absorption in the [[Si]{}1394,1403Å]{} doublet in PG0843+516 and SDSS1228+1040, reminiscent to similar high-ionisation lines seen in the *HST* spectra of white dwarfs in cataclysmic variables. We suspect that these lines originate in hot gas close to the white dwarf, well within the sublimation radius.
author:
- |
    B.T. Gänsicke$^1$, D. Koester$^2$, J. Farihi$^3$, J. Girven$^1$, S.G. Parsons$^1$, E. Breedt$^1$\
    $^{1}$ Department of Physics, University of Warwick, Coventry CV4 7AL, UK\
    $^{2}$ Institut für Theoretische Physik und Astrophysik, University of Kiel, 24098 Kiel, Germany\
    $^{3}$ Department of Physics Astronomy, University of Leicester, Leicester LE1 7RH, UK
date: 'Accepted 2005. Received 2005; in original form 2005'
title: 'The chemical diversity of exo-terrestrial planetary debris around white dwarfs'
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Stars: individual: PG0843+516, PG1015+161, SDSSJ122859.93+104032.9, GALEXJ193156.8+011745 – white dwarfs – circumstellar matter – planetary systems

Introduction
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Most of our current insight into the interior structure of exo-planets 