  The accretion of comets onto DA white dwarfs can produce observable metal
absorption lines. We show here that comet systems around the progenitor main
sequence star are vulnerable to being lost during asymptotic giant branch mass
loss, if the mass loss is sufficiently asymmetric to impart modest linear
momentum to the white dwarf. This may have bearing on the frequency of
observation of heavy elements in white dwarf stars, and on inferences regarding
the frequency of comet systems, if the imparted linear velocities of white
dwarfs can be estimated.
