  The Neptune Trojans are the most recently discovered population of small
bodies in the Solar System. To date, only eight have been discovered, though it
is thought likely that the total population at least rivals that of the
asteroid belt. Their origin is still the subject of some debate. Here, we
detail the results of dynamical studies of two Neptune Trojans, 2001 QR322 and
2008 LC18. We find that both objects lie very close to boundaries between
dynamically stable and unstable regions, with a significant probability that
either or both of the objects are actually unstable on timescales of a few
hundred million years. Such instability supports the idea that at least these
two Neptune Trojans are dynamically captured objects, rather than objects that
formed in situ. This that does not, however, rule out the possibility that
these two objects were captured during Neptune's proposed post-formation
migration, and have remained as Trojans ever since.
