Aims. We report on evidence of the inhomogeneity (multiplicity) of the stellar population in the Galactic globular cluster (GC) NGC 3201, which is irregularly reddened across its face. Methods. We carried out a more detailed and careful analysis of our recently published new multi-color photometry in a wide field of the cluster with particular emphasis on the U band. Results. Using the photometric data corrected for differential reddening, we found for the first time two key signs of the inhomogeneity in the cluster's stellar population and of its radial variation in the GC. These are (1) an obvious trend in the color-position diagram, based on the (U − B) color-index, of red giant branch (RGB) stars, which shows that the farther from the cluster's center, the bluer on average the (U − B) color of the stars is; and (2) the dependence of the radial distribution of sub-giant branch (SGB) stars in the cluster on their U magnitude, where brighter stars are less centrally concentrated than their fainter counterparts at a confidence level varying between 99.2% and 99.9% depending on the color-index used to select the stars. The same effects were recently found by us in the GC NGC 1261. However, contrary to NGC 1261, we are not able to unambiguously suggest which of the sub-populations of SGB/RGB stars can be the progenitor of blue and red horizontal branch stars of the cluster. Apart from M4, NGC 3201 is another GC very probably with an inhomogeneous stellar population, which has essentially lower mass than the most massive Galactic GCs where multiple stellar populations were unambiguously detected for the first time.

Aims. This paper aims at further studying one of the nearby Galactic globular clusters (GCs), NGC 3201. It is known to experience notable irregular variability of reddening across its face. By relying on our previous studies and findings and by developing them, we focus on the brighter sequences of the color-magnitude diagram (CMD) and on the cluster's characteristics. Methods. We carried out and analyzed new multi-color photometry of NGC 3201 in UBVI reaching below the turnoff point in all passbands in a fairly large cluster field, about 14 × 14 . To achieve more reliable results and conclusions, we reduced the negative impact of the irregularly varying reddening and contamination by field stars. Results. With this aim we first estimated mean reddening in different zones of the studied cluster field and then took its variations into account, by reducing them to the same level. We estimated metallicity of NGC 3201 using a new metallicity indicator related to U-based CMDs, recently proposed by us. We find [Fe/H] ZW = −1.54 ± 0.12 dex, which falls between extreme estimates of the cluster's metallicity obtained using different methods or indicators. Also, the location of the RGB bump on the branch corresponds to [Fe/H] ZW = −1.46 ± 0.15 dex. We isolate 73 probable blue straggler (BS) candidates, the largest population found in NGC 3201 so far. They are more centrally concentrated than the lower red giants at the 99.2% level. Their position in the two-color diagram assumes that presumably none of them belongs to BSa of collisional origin. The luminosity function (LF) of the RGB and its features in the low part of the branch are examined and discussed. We also resolve some of the contradictory results of previous publications.

Aims. This work studies in more detail the stellar population, including its photometric properties and characteristics, in the rarely studied southern Galactic globular cluster NGC 1261. We focus on the brighter sequences of the cluster's color-magnitude diagram (CMD). Like in our previous works, we rely upon photometry in several passbands to achieve more reliable results and conclusions. Methods. We carried out and analyzed new multi-color photometry of NGC 1261 in UBVI reaching below the turnoff point in all passbands in a fairly extended cluster field, about 14 × 14 . Results. We found several signs of the inhomogeneity ("multiplicity") in the stellar population. The most prominent of them are: (1) the dependence of the radial distribution of sub-giant branch (SGB) stars in the cluster on their U magnitude, with brighter stars less centrally concentrated at the 99.9% level than their fainter counterparts; (2) the dependence of the location of red giant branch (RGB) stars in the U-(U − B) CMD on their radial distance from the cluster center, with the portion of stars bluer in the (U − B) color increasing towards the cluster outskirts. Additionally, the radial variation of the RGB luminosity function in the bump region is suspected. We assume that both the SGB stars brighter in the U and the RGB stars bluer in the (U − B) color are probably associated with blue horizontal branch stars, because of a similarity in their radial distribution in the cluster. We estimated the metalicity of NGC 1261 from the slope of the RGB in U-based CMDs and the location of the RGB bump on the branch. These metallicity indicators give [Fe/H] ZW = −1.34 ± 0.16 dex and [Fe/H] ZW = −1.41 ± 0.10 dex, respectively. We isolated 18 probable blue straggler candidates. They are more centrally concentrated than the lower red giants of comparable brightness at the 97.9% level. Their photometric characteristics imply that their majority is not consistent with the collisional origin. We also reliably isolated the asymptotic giant branch (AGB) clump and estimated the parameter ΔV clump ZAHB = 1.01 ± 0.06, that is the difference between the V-levels of the zero age HB and the clump.

Aims. We investigate the new and still poorly studied matter of so-called multiple stellar populations (MSPs) in Galactic globular clusters (GGCs). Studying MSPs and their accumulated data can shed more light on the formation and evolution of GGCs and other closely related fundamental problems. We focus on the strong relation between the radial distribution of evolutionary homogeneous stars and their U-based photometric characteristics in the nearby GGC NGC 6752 and compare this with a similar relation we found in NGC 3201 and NGC 1261. Methods. We use our new multi-color photometry in a fairly wide field of NGC 6752, with particular emphasis on the U band and our recent and already published photometry made in NGC 3201 and NGC 1261. Results. We found and report here for the first time a strong difference in the radial distribution between the sub-populations of red giant branch (RGB) stars that are bluer and redder in color (U −B), as well as between sub-giant branch (SGB) stars brighter and fainter in the U-magnitude in NGC 6752. Moreover, the fainter SGB and redder RGB stars are similarly much more centrally concentrated than their respective brighter and bluer counterparts. Virtually the same applies to NGC 3201. We find evidence in NGC 6752 as in NGC 3201 that a dramatic change in the proportion of the two sub-populations of SGB and RGB stars occurs at a radial distance close to the half-mass radius, R h , of the cluster. These results are the first detections of the radial trend of the particular photometric properties of stellar populations in GGCs. They imply a radial dependence of the main characteristics of the stellar populations in these GGCs, primarily of the abundance, and (indirectly) presumably of the kinematics.