References for the numbering systems

Col	Reference

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

R1	van den Bergh S. 1970, Astron. Astrophys. 9, 209

R2	Hoag A.A., et al. 1961, Publ. US Nav. Obs. 17, 347  (pe)

R3	Hoag A.A., et al. 1961, Publ. US Nav. Obs. 17, 347  (pg)

R4	Sharov A.S. 1968, Soobs. Gos. Astr. Inst. Shternberga 158, 43

R5	Crinklaw G., Talbert F.D. 1991, Publ. Astr. Soc. Pacific 103, 536

R6	Janes K.A., Hoq S. 2011, Astron. J. 141, 92

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