The M5 RR Lyrae population

We present V- and I-band CCD photometry of the central regions of the globular cluster M5. Our observations, spanning a period of just over one year, cover a magnitude range from above the horizontal branch to ∼ 1 mag below the turn-off, allowing us to determine accurate light-curves for 49 RR Lyraes, including three newly discovered variables. Our colour-magnitude data show that the horizontal branch narrows from ∼ 0.4 mag (full-width) on the red side of the instability strip to ∼ 0.2 mag in the blue, with the majority of the RR Lyraes having mean magnitudes within a ∼0.25 mag range. A few stars, however, lie up to 0.35 mag above the lower-luminosity edge of the horizontal branch, and we suggest that these stars may be evolving towards the asymptotic giant branch. We have combined our observations with data from the literature to determine the secular period variations of the more thoroughly surveyed stars in the sample. Finally, we compare the Oosterhoff type I M5 variables with the RR Lyraes in the type II cluster M68.