  The star HR 8799 hosts one of the largest known debris discs and at least
four giant planets. Previous observations have found evidence for a warm belt
within the orbits of the planets, a cold planetesimal belt beyond their orbits
and a halo of small grains. With the infrared data, it is hard to distinguish
the planetesimal belt emission from that of the grains in the halo. With this
in mind, the system has been observed with ALMA in band 6 (1.34 mm) using a
compact array format. These observations allow the inner edge of the
planetesimal belt to be resolved for the first time. A radial distribution of
dust grains is fitted to the data using an MCMC method. The disc is best fit by
a broad ring between $145^{+12}_{-12}$ AU and $429^{+37}_{-32}$ AU at an
inclination of $40^{+5}_{-6}${\deg} and a position angle of
$51^{+8}_{-8}${\deg}. A disc edge at ~145 AU is too far out to be explained
simply by interactions with planet b, requiring either a more complicated
dynamical history or an extra planet beyond the orbit of planet b.
