CCD photometry of the galactic globular cluster M13

In this paper, we present a B-V CCD colour-magnitude diagram (CMD) of the globular cluster M13 (NGC 6205), reaching down about 2 mag below the main sequence turn-off, based on a new, independent photometric calibration. The most notable features of the CMD, including about 5500 stars, are (i) the long extension of the blue tail of the horizontal branch (HB), which can be traced down to V similar to 19 and (moreover) shows the presence of two gaps; and (ii) the so-called bump on the red giant branch which is well-detected at V=14.75+/-0.05, using both the differential and integral luminosity functions. No similar features have ever been detected in any previous ground-based photometry of this cluster. A direct comparison between the CMD of M13 and M3, calibrated during the same run with the same apparatus, sets an upper limit of about 1.5 Gyr on their age difference, too small to explain the striking differences between their HB morphologies.