1 1951ApJ...114..492R Roman
 2 1953ApJ...118..318M Morgan et al.          and 1955ApJS....2...41M
 3 1956VA......2.1124M Morgan & Harris
 4 1956ApJS....2..389H Hiltner
 5 1961AbaOB..26...35K Kharadze & Bartaya
 6 1962JO.....45...57B Barbier
 7 1965ApJS...12..215H Hoag & Applequist
 8 1968PASP...80..290W Walker & Hodge
 9 1971AZh....48..755R Raznik                 and 1969AZh....46..837R
10 1970ApJ...161L.149W Walborn                and 1973AJ.....78.1067W
11 1976ApJS...30..491H Henize
12 1977PW&SO...2...71S Stephenson & Sanduleak
13 1981PASP...93..441F Forbes
14 1990A&AS...85.1069S Sato & Kuji
15 1995ApJ...454..151M Massey et al.          Cat. J/ApJ/545/151
16 2000A&A...356..118W Wang & Hu
17 2004A&A...415..145B Boeche et al.          Cat. J/A+A/415/145
18 2004AN....325..380N Negueruela