References for the numbering systems

Col	Reference

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

R1	Yilmaz F. 1970, Astron. Astrophys. 8, 213

R2	Viskum M., et al. 1997 Astron. Astrophys. 328, 158

R3	Lata S. et al. 2014, New Astron. 26, 77 

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