On the triple-star origin of the planetary nebula Sh 2-71
  Recent studies have indicated that triple star systems may play a role in the
formation of an appreciable number of planetary nebulae, however only one
triple central star is known to date (and that system is likely too wide to
have had much influence on the evolution of its component stars). Here, we
consider the possibility that Sh 2-71 was formed by a triple system which has
since broken apart. We present the discovery of two regions of emission,
seemingly aligned with the proposed tertiary orbit (i.e. in line with the axis
formed by the two candidate central star systems previously considered in the
literature). We also perform a few simple tests of the plausibility of the
triple hypothesis based on the observed properties (coordinates, radial
velocities, distances and proper motions) of the stars observed close to the
projected centre of the nebula, adding further support through numerical
integrations of binary orbits responding to mass loss. Although a number of
open questions remain, we conclude that Sh 2-71 is currently one of the best
candidates for planetary nebula formation influenced by triple-star
interactions.
