UBV Photometry of RR Lyrae Variables in the Globular Cluster M4
M4, one of the closest globular clusters to the Sun, presents some peculiar features that make it very interesting to study. The first C-M diagram, published by Greenstein (1939), showed a well populated blue-horizontal branch, a characteristic which usually, but not always, indicates low metal abundance (Hartwick 1968). Kinman (1959) found however that it contains strong lined red giants and classified it as type A, indicating normal metal abundance. More recent C-M diagrams (Moshkalev 1975, Alcaino 1975, Lee 1977, Lloyd Evans 1977) substantially confirmed both these characteristics.One of the major problems in the study of this cluster is the high and possibly non-uni form reddening, due to its location behind the Scorpius-Ophiuchus dark clouds.