nomy and Astrophysics 375 (1):  30–9, doi:10.1051/0004-6361:20010845, Bibcode: 2001A&A...375...30M Miszalski, B.; Jones, D.; Rodríguez-Gil, P.; Boffin, H. M. J.; Corradi, R. L. M.; Santander-García, M. (2011), "Discovery of close binary central stars in the planetary nebulae NGC 6326 and NGC 6778", Astronomy and Astrophysics 531:  A158, doi:10.1051/0004-6361/201117084, Bibcode: 2011A&A...531A.158M Moore, S. L. (October 2007), "Observing the Cat's Eye Nebula", Journal of the British Astronomical Association 117 (5):  279–80, Bibcode: 2007JBAA..117R.279M Morris, M. (1990), "Bipolar asymmetry in the mass outflows of stars in transition", in Mennessier, M.O.; Omont, Alain, From Miras to planetary nebulae: which path for stellar evolution?, Montpellier, France, September 4–7, 1989 IAP astrophysics meeting: Atlantica Séguier Frontières, pp. 526–30, ISBN 978-2-86332-077-8, http://books.google.com/?id=qTZld_-Y5qYC Osterbrock, Donald E.; Ferland, G. J. (2005), Ferland, G. J., ed., Astrophys