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

Accurate BVRI photoelectric photometry and radial velocities derived from 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 the four stars. New epochs are derived from the present data and new periods are calculated for three of the variables using a large data base available in the literature. Particular attention is given to V15. This star presents a small-amplitude radial velocity curve associated with a light curve that seems to be switching from ab- to c-type, suggesting that it might be progressively changing its mode of pulsation while evolving towards the blue part of the RR Lyrae instability strip. An alternative possibility might be that V15 is affected by a strong Blazhko effect. The available spectra confirm the occurrence of non-linear 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 sudden 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 produces a broadening and a doubling of the metal lines which results in a lower accuracy of the corresponding radial velocity measurements. Some support is given to the hypothesis that the shock may begin high in the atmosphere where the cores of the H lines are formed.