On the formation of DA white dwarfs with low hydrogen contents:   Preliminary Results
  Systematic photometric and asteroseismological studies in the last decade support the belief that white dwarfs in the solar neighborhood harbor a broad range of hydrogen-layer contents. The reasons behind this spread of hydrogen-layer masses are not understood and usually misunderstood. In this work we present, and review, the different mechanisms that can (or cannot) lead to the formation of white dwarfs with a broad range hydrogen contents. 
M. M. Miller Bertolami, L. G. Althaus, and A. H. C\'orsico
astro-ph.SR