BVRI PHOTOELECTRIC PHOTOMETRY AND RADIAL-VELOCITY DATA FOR 4 RR LYRAESTARS IN THE GLOBULAR-CLUSTER NGC-6121 - THE STRANGE CASE OF V15

Accurate BVRI photoelectric photometry and radial velocities derived f rom high-resolution spectra (0.25 and 0.19 angstrom) are presented for four RR Lyrae variables, namely V2, V15, V29 and V42, in the globular cluster M4 (NGC 6121). The data cover the entire pulsation cycle of t he four stars. New epochs are derived from the present data and new pe riods are calculated for three of the variables using a large data bas e available in the literature. Particular attention is given to V15. T his star presents a small-amplitude radial velocity curve associated w ith a light curve that seems to be switching from ab- to c-type, sugge sting that it might be progressively changing its mode of pulsation wh ile evolving towards the blue part of the RR Lyrae instability strip. An alternative possibility might be that V15 is affected by a strong B lazhko effect. The available spectra confirm the occurrence of non-lin ear phenomena, such as shock waves, during the ascending branch of the light curve of the ab-type variables. The effect of the shock is a su dden emission that appears in all the H lines, in correspondence with a hump in the light curve of the ab-type variables. The shock also pro duces a broadening and a doubling of the metal lines which results in a lower accuracy of the corresponding radial velocity measurements. So me support is given to the hypothesis that the shock may begin high in the atmosphere where the cores of the H lines are formed.