References for the numbering systems

Col	Reference

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

R1	Hassan S.M. 1976, Astron. Astrophys. Suppl. 26, 9

R2	Pound M.W., Janes K.A. 1986, Publ. Astron. Soc. Pacific 98, 210

R3	Kim S.-L., et al. 1999, Astron. Astrophys. 348, 795

R4	Twarog B.A., et al. 2006, Astron. J. 132, 299

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