References for the numbering systems

Col	Reference

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

R1	Hoag A.A., et al. 1961, Publ. US Nav. Obs. 17, 347  (pe)

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

R3	Svolopoulos S.N. 1953, Mon. Not. R. Astr. Soc. 113, 758

R4	Sanders W.L., Schroeder R. 1980, Astron. Astrophys. 88, 102

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