Knots in the outer shells of the planetary nebulae IC 2553 and NGC 5882
  We present images and high-resolution spectra of the planetary nebulae IC 2553 and NGC 5882. Spatio-kinematic modeling of the nebulae shows that they are composed of a markedly elongated inner shell, and of a less aspherical outer shell expanding at a considerably higher velocity than the inner one. Embedded in the outer shells of both nebulae are found several low-ionization knots. In IC 2553, the knots show a point-symmetric distribution with respect to the central star: one possible explanation for their formation is that they are the survivors of pre-existing point-symmetric condensations in the AGB wind, a fact which would imply a quite peculiar mass-loss geometry from the giant progenitor. In the case of NGC 5882, the lack of symmetry in the distribution of the observed low-ionization structures makes it possible that they are the result of in situ instabilities. 
R.L.M. Corradi (1), D.R. Goncalves (2), E. Villaver (2), A. Mampaso   (2) and M. Perinotto (3) ((1) Isaac Newton Group of Te...