CAPUTO, F.; CASTELLANI, V.; QUARTA, M.L. Reddening, distance modulus and age of the globular cluster NGC 6121 (M4) from the properties of RR Lyrae variables" [online]. Astronomy and Astrophysics 143, 1985, s. 8 až 12 . [cit. 2009-04-29]. Dostupné online. (anglicky)[nedostupný zdroj]