Johnson, H. L.; Morgan, W. W. (1953). ”Fundamental stellar photometry for standards of spectral type on the Revised System of the Yerkes Spectral Atlas”. The Astrophysical Journal 117: sid. 313. doi:10.1086/145697.
Keenan, Philip C.; McNeil, Raymond C. (1989). ”The Perkins Catalog of Revised MK Types for the Cooler Stars”. The Astrophysical Journal Supplement Series 71: sid. 245. doi:10.1086/191373.