A distance estimate based on angular expansion for the planetary nebula
  NGC 6881
  In this paper, we report on high angular resolution radio observations of the
planetary nebula NGC 6881 obtained with the Very Large Array at a wavelength of
6 cm. The emission appears to be the superposition of a roundish core and a
point-symmetric bipolar structure elongated along a position angle of about
145$^\circ$. This is strongly reminiscent of the morphology seen in H$\alpha$
and [NII] images. A comparison between VLA observations obtained in 1984 and
1994 clearly reveals the expansion of the core of the nebula, at a rate of 2.1
$\pm$ 0.7 mas yr$^{-1}$. Assuming that the expansion velocity in the plane of
the sky (determined from these measurements) and the expansion velocity along
the line of sight (estimated from optical spectroscopy available in the
literature) are equal, we find a distance to NGC 6881 of 1.6 $\pm$ 0.5 kpc
$\pm$ 0.3 kpc, where the first error reflects the uncertainty on the expansion,
and the second error comes from the potential difference between pattern and
material speeds. This distance is compatible with (but does not necessarily
imply) an association of NGC 6881 with the nearby HII region Sh 2-109 and, more
generally, the Cygnus star-forming region.
