  We report on a search for evidence of binarity in Far-Ultraviolet
Spectroscopic Explorer (FUSE) observations of DAO white dwarfs. Spectra
recorded by FUSE are built up from a number of separate exposures. Observation
of changes in the position of photospheric heavy element absorption lines
between exposures, with respect to the stationary interstellar medium lines,
would reveal radial velocity changes - evidence of the presence of a binary
system. This technique is successful in picking out all the white dwarfs
already known to be binaries, which comprise 5 out of the sample of 16, but
significant radial velocity shifts were found for only one additional star, Ton
320. This object is also known to have an infrared excess. DAOs can be
separated broadly into low or normal mass objects. Low mass white dwarfs can be
formed as a result of binary evolution, but it has been suggested that the
lower mass DAOs evolve as single stars from the extended horizontal branch, and
we find no evidence of binarity for 8 out of the 12 white dwarfs with
relatively low mass. The existence of higher mass DAOs can also be explained if
they are within binary systems, but of the four higher mass stars in the sample
studied, PG 1210+533 and LB 2 do not exhibit significant radial velocity
shifts, although there were only two exposures for the former object and the
latter has an infrared excess.
