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Gray, R. O.; Garrison, R. F. (1989-07-01). "The Late A-Type Stars: Refined MK Classification, Confrontation with Stroemgren Photometry, and the Effects of Rotation". The Astrophysical Journal Supplement Series. 70: 623. Bibcode:1989ApJS...70..623G. doi:10.1086/191349. ISSN0067-0049.