References for the numbering systems

Col	Reference

---	---------

R1	Hawarden T.G. 1974, Mon. Not. R. Astr. Soc. 169, 539

R2	Anthony-Twarog B.J., Twarog B.A. 1989, Astron. J. 97, 1048

R3	Piatti A.E., et al. 1998, Astrophys. J. Suppl. 116, 263

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