↑ ab et cP. A. Jennens et H. L. Helfer, « A new photometric metal abundance and luminosity calibration for field G and K giants », Monthly Notices of the Royal Astronomical Society, (DOI10.1093/mnras/172.3.667, Bibcode1975MNRAS.172..667J)
↑Philip C. Keenan et Raymond C. McNeil, « The Perkins catalog of revised MK types for the cooler stars », Astrophysical Journal Supplement Series, (DOI10.1086/191373, Bibcode1989ApJS...71..245K)