van de Steene, G.C.; Jacoby, G.H.; Praet, C.; Ciardullo, R.; Dejonghe, H. (2006). “Distance determination to NGC 55 from the planetary nebula luminosity function”. Astronomy and Astrophysics. 455 (3): 891–896. Bibcode:2006A&A...455..891V. doi:10.1051/0004-6361:20053475.
van de Steene, G.C.; Jacoby, G.H.; Praet, C.; Ciardullo, R.; Dejonghe, H. (2006). “Distance determination to NGC 55 from the planetary nebula luminosity function”. Astronomy and Astrophysics. 455 (3): 891–896. Bibcode:2006A&A...455..891V. doi:10.1051/0004-6361:20053475.