Infrared Excess around

We present ISOCAM 7 µm and 15 µm observations of 12 nearby white dwarfs, 6 of which have been found to have metals such as Ca, Mg and Fe in their pho-tospheres. Our purpose was to search for an excess of infrared emission above the stellar photospheres. We find that none of the white dwarfs other than G29−38 shows a detectable infrared excess and this places strong constraints on the existence of a dusty disk around these stars. We conclude that ongoing accretion of the interstellar medium seems an unlikely explanation for the existence of metals in the photo-spheres of cool hydrogen atmosphere white dwarfs. The excess associated with G29−38 is 3.8±1.0 mJy and 2.9±0.6 mJy at 7 µm and 15 µm respectively. The broadband spectrum of this star strengthens the hypothesis that the infrared excess arises from a disk of particulate matter surrounding the white dwarf rather than from a cool brown dwarf companion.