Hill, P. W.; Lynas-Gray, A. E. (September 1977), "UBV photometry and MK spectral classification of northern early-type stars at intermediate galactic latitudes.", Monthly Notices of the Royal Astronomical Society, 180 (4): 691–702, Bibcode:1977MNRAS.180..691H, doi:10.1093/mnras/180.4.691.
Hill, P. W.; Lynas-Gray, A. E. (September 1977), "UBV photometry and MK spectral classification of northern early-type stars at intermediate galactic latitudes.", Monthly Notices of the Royal Astronomical Society, 180 (4): 691–702, Bibcode:1977MNRAS.180..691H, doi:10.1093/mnras/180.4.691.