CCD-PHOTOMETRY OF GALACTIC GLOBULAR-CLUSTERS .3. NGC-6171

B and V CCD photometry is presented for 1565 stars in the low-concentration globular cluster NGC 6171 = M107. The main conclusions drawn from the analysis of the colour-magnitude diagram (CMD) here presented are the following. (i) The overall distribution of stars in the CMD confirms the results presented by previous studies for the bright branches, while a new reliable main-sequence (MS) locus is determined down to about three mag fainter than the turn-off (TO) which is located at V = 19.20 +/- 0.10 and B - V = 0.86 +/- 0.03. (ii) A specific photometric procedure applied to star images in the very central region of the cluster suggests the existence of a population of about 20 blue straggler (BS) candidates. If confirmed, BSs would thus be more centrally concentrated than normal stars as already found in other globular clusters of low concentration (NGC 5053, NGC 5466). (iii) The mean metallicity [from DELTA-V, (B - V)o,g, S] is [Fe/H] = -0.89 +/- 0.2, and the very small intrinsic widths of the various branches indicate very low upper limits for the abundance spread amongst cluster members. (iv) The so-called 'RGB-bump', predicted to exist in the luminosity function of the red giant branch by theoretical evolutionary models, has been detected at V = 15.85 +/- 0.05. (v) There is evidence for a difference in the spatial distribution of stars belonging to the red giant branch (RGB) and the horizontal branch (HB), the red giants being more centrally concentrated. (vi) The difference in luminosity between the horizontal branch and the TO is DELTA-V(HB)TO = 3.55 +/- 0.11 - totally consistent with the mean value obtained by Buonanno et al. (1989) for 18 other galactic globulars and the figure 16(-2)+4 Gyr currently derived for the age of typical halo clusters (Fusi Pecci & Cacciari 1990).