Metals in cool DA white dwarfs

We used the HIRES echelle spectrograph on the Keck telescope to search for metals in the 3700-5200 Angstrom spectra of white dwarfs with hydrogen-rich photospheres. The most popular models that account for the presence of met als are interstellar accretion and comet impact. The data are not easily re conciled with either model, but binarity might play a role. We comment on t he implications for Oort-Like comet clouds and certain types of planets aro und main-sequence stars.