ON THE GIANT, HORIZONTAL AND ASYMPTOTIC BRANCHES OF GALACTIC GLOBULAR-CLUSTERS .5. CCD PHOTOMETRY OF NGC-1261

BV CCD photometry is presented for 3352 stars (18 variables) in the gl obular cluster NGC 1261. The main conclusions drawn from the structure of the colour-magnitude diagram (CMD) are as follows. (i) The locatio n and structure of the main branches agree with those found in previou s studies, confirming the existence of a well-defined blue horizontal branch (HB) tail in addition to the large number of red HB stars. (ii) The mean metallicity derived from CMD indicators calibrated with the Zinn & West scale is [Fe/H] = -1.4 +/- 0.2, and the very small intrins ic width of the various branches yields very low upper limits (DELTA[F e/H] less-than-or-equal-to 0.06, DELTA[CNO] less-than-or-equal-to 0.2) for any chemical abundance spread. (iii) The 'RGB bump' is detected a t V = 16.70 +/- 0.05 as a clump of stars in the luminosity function of the red giant branch. (iv) V(HB) = 16.70 +/- 0.04 and V(TO) = 20.14 /- 0.10 yield DELTAV(HB)TO = 3.44 +/- 0.12. A detailed comparison with the main CMD loci of NGC 288 (a cluster that has similar metallicity but a totally blue HB) throws doubt on the hypothesis that a clear dif ference in age (NGC 288 being older) is the 'second parameter' necessa ry to explain the observed differences. (v) Some evidence is found for differences in the spatial distribution of the red and blue HB stars that is worthy of further analysis.