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Waagen, E.; Evans, R.; Cragg, T.; Dunbar, S.; Helin, E.; Barucci, M.; Swanson, S. (1984). "Supernova in NGC 991". International Astronomical Union Circular (3979): 1. Bibcode:1984IAUC.3979....1W.
Wheeler, J. C.; Levreault, R. (July 1985). "The peculiar type I supernova in NGC 991". Astrophysical Journal. 294: L17 –L20. Bibcode:1985ApJ...294L..17W. doi:10.1086/184500.