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Schild (1983)
CCD photometry of M67 stars useful as BVRI standards
http://adsabs.harvard.edu/abs/1983PASP...95.1021S

Chevalier & Ilovaisky (1991)
Cousins BVRI CCD photometry of stars in the M67 'dipper asterism'
http://adsabs.harvard.edu/abs/1991A%26AS...90..225C

Montgomery et al. (1993)
CCD photometry of the old open cluster M67
http://adsabs.harvard.edu/abs/1993AJ....106..181M

Natali et al. (1994)
The use of the (B-I) color index and applications of the (B-I) versus (B-V) relationship
http://adsabs.harvard.edu/abs/1994A%26A...289..756N