Stars with Unusual Compositions: Carbon and Oxygen in Cool White Dwarf   Stars
  I present a broad historical review on cool white dwarf stars with carbon and/or oxygen detected at the photosphere. This book chapter covers the observational signatures, physical properties, and evolution of DQ, DBQ and Hot DQ white dwarfs, and also present an overview of the main challenges that future investigations of these types of object should try to address. 
P. Dufour
astro-ph.SR