  The aim of the Degenerate Objects around Degenerate Objects (DODO) survey is
to search for very low mass brown dwarfs and extrasolar planets in wide orbits
around white dwarfs via direct imaging. The direct detection of such companions
would allow the spectroscopic investigation of objects with temperatures much
lower (< 500 K) than the coolest brown dwarfs currently observed. These
ultra-low mass substellar objects would have spectral types > T8.5 and so could
belong to the proposed Y dwarf spectral sequence. The detection of a planet
around a white dwarf would prove that such objects can survive the final stages
of stellar evolution and place constraints on the frequency of planetary
systems around their progenitors (with masses between 1.5 - 8 solar masses,
i.e., early B to mid F). This paper presents the results of a multi-epoch J
band common proper motion survey of 23 nearby equatorial and northern
hemisphere white dwarfs. We rule out the presence of any common proper motion
companions, with limiting masses determined from the completeness limit of each
observation, to 18 white dwarfs. For the remaining five targets, the motion of
the white dwarf is not sufficiently separated from the non-moving background
objects in each field. These targets require additional observations to
conclusively rule out the presence of any common proper motion companions. From
our completeness limits, we tentatively suggest that < 5% of white dwarfs have
substellar companions with effective temperatures > 500 K between projected
physical separations of 60 - 200 AU.

Earth and Planetary Astrophysics;Solar and Stellar Astrophysics