Establishing the relationships between those groups of white dwarfs whose compositions are dominated by hydrogen or helium is crucial to understanding the evolution of the population and the physical processes that determine this. Of particular importance are accurate measurements of photospheric composition which can tell us how the helium and heavy element content of white dwarfs may change as the stars cool.
Photometric observations leave several major questions unanswered.
The availability of spectroscopic information from EUVE has provided us with an opportunity of searching directly for helium in the atmospheres of H-rich white dwarfs in the crucial region of the HeII Lyman absorption series between 304 Angstroms and the limit at 228 Angstroms. Those stars studied can be divided into two distinct groups - those with and without heavy element opacity. If heavy elements are present, the stars can only be properly studied using complex line-blanketed models with a comprehensive selection of opacity sources. Approximately a dozen EUVE spectra of stars without heavy element opacity have been carefully examined for spectroscopic traces of helium. Only two stars show clear evidence of photospheric helium, REJ0720-317 and GD50.