References for the numbering systems

Col	Reference

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R1	Steppe H. 1974, Astron. Astrophys. Supp. 15, 91

R2	McCuskey S.W., Houk N. 1964, Astron. J. 69, 412

R3	Phelps R.L., Janes K.A. 1994, Astrophys. J. Suppl. 90, 31

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