Carbon Deficiency in Externally-Polluted White Dwarfs: Evidence for   Accretion of Asteroids
  Existing determinations show that n(C)/n(Fe) is more than a factor of 10 below solar in the atmospheres of three white dwarfs that appear to be externally-polluted. These results are not easily explained if the stars have accreted interstellar matter, and we re-interpret these measurements as evidence that these stars have accreted asteroids of a chrondritic composition. 
M. Jura (UCLA)