Keenan, Philip C.; McNeil, Raymond C. (1989), The Perkins Catalog of Revised MK Types for the Cooler Stars, The Astrophysical Journal Supplement Series, 71: 245, Bibcode:1989ApJS...71..245K, doi:10.1086/191373.
Keenan, Philip C.; McNeil, Raymond C. (1989), The Perkins Catalog of Revised MK Types for the Cooler Stars, The Astrophysical Journal Supplement Series, 71: 245, Bibcode:1989ApJS...71..245K, doi:10.1086/191373.