times. However, analysis will remain hindered until more reliable information on white dwarf masses and distances becomes available. Until then, the conclusions in this work must be regarded as preliminary.

Acknowledgements
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We thank an anonymous referee for helpful comments and corrections. PG wishes to thank the Space Telescope Science Institute for its kind hospitality. This work was supported by NSF grant AST05-07514 and NASA grants NAG5-12067 and NNG04GE78 to Villanova University. This research was based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. [*[FUSE]{}*]{} is operated for NASA by the Johns Hopkins University under NASA contract NAS5-32985.

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