We present the results from V I CCD time-series photometry of the globular cluster M13 (NGC 6205). From the Fourier decomposition of the light curves of RRab and RRc stars we found an average metallicity of [Fe/H]zw = -1.58 ± 0.09. The distance to the cluster was estimated as 7.1 ± 0.1 kpc from independent methods related to the variable star families RR Lyrae, SX Phe and W Virginis, from the luminosity of the theoretical ZAHB and from the orbit solution of a newly discovered contact binary star. The RR Lyrae pulsation modes are segregated by the red edge of the first overtone instability strip in this OoII type cluster. A membership analysis of 52,800 stars in the field of the cluster is presented based on Gaia-DR2 proper motions which enabled the recognition of 23,070 likely cluster members, for 7,630 of which we possess V I photometry. The identification of member stars allowed the construction of a clean CMD and a proper ZAHB and isochrone fitting, consistent with a reddening, age and distance of 0.02 mag, 12.6 Gyrs and 7.1 kpc respectively. We report seven new variables; one RRc, two SX Phe stars, three SR and one contact binary. V31 presents double-mode nature and we confirm V36 as RRd. Fifteen variable star candidates are also reported. The analysis of eighteen stars in the field of the cluster, reported as RR Lyrae from the Gaia-DR2 data base reveals that at least seven are not variable. We noted the presence of a high velocity star in the field of the cluster.

We present an analysis of VI CCD time-series photometry of the Oo II type globular cluster M92. The variable star population of the cluster is studied with the aim of revising their classifications, identifications, frequency spectra and to select indicators of the parental cluster metallicity and distance. The Fourier decomposition of RR Lyrae light curves lead to the estimation of mean [Fe/H] spec = −2.20 ± 0.18, and distance of 8.3 ± 0.2 kpc. Four new variables are reported; one RRd (V40), a multimode SX Phe (V41), a SR (V42) and one RRc (F1) that is most likely not a cluster member. The AC nature of V7 is confirmed. The double mode nature of the RRc star V11 is not confirmed and its amplitude modulations are most likely due to the Blazhko effect. Two modes are found in the known RRc variable V13. It is argued that the variable V30, previously classified as RRab is, in fact, an BL Her-type star not belonging to the cluster. Making use of the Gaia-DR2 proper motions, we identified 5012 stars in the field of the cluster, that are very likely cluster members, and for which we possess photometry, enabling the production of a refined Colour-Magnitude Diagram. This also allowed us to identify a few variable stars that do not belong to the cluster. The RR Lyrae pulsation modes on the HB are cleanly separated by the first overtone red edge, a common feature in all Oo II type clusters.

High-resolution Hubble Space Telescope optical observations have been used to analyze the stellar population and the structure of the poorly investigated bulge globular cluster NGC 6316. We constructed the first high-resolution reddening map in the cluster direction, which allowed us to correct the evolutionary sequences in the color–magnitude diagram (CMD) for the effects of differential reddening. A comparison between the CMDs of NGC 6316 and 47 Tucanae revealed strikingly similar stellar populations, with the two systems basically sharing the same turnoff, subgiant branch, and horizontal branch morphologies, indicating comparable ages. The red giant branch in NGC 6316 appears slightly bluer than in 47 Tucanae, suggesting a lower metal content. This has been confirmed by the isochrone fitting of the observed CMD, which provided us with updated values of the cluster age, distance, average color excess, and metallicity. We estimated an absolute age of 13.1 ± 0.5 Gyr, consistent with the age of 47 Tucanae, an average color excess E(B − V) = 0.64 ± 0.01, and a true distance modulus (m − M)0 = 15.27 ± 0.03 that sets the cluster distance at 11.3 kpc from the Sun. In addition, the photometric estimate of the cluster metallicity suggests [Fe/H] ≈ −0.9, which is ∼0.2 dex smaller than that of 47 Tucanae. We also determined the gravitational center and the density profile of the system from resolved stars. The latter is well reproduced by a King model. Our results confirm that NGC 6316 is another extremely old relic of the assembly history of the Galaxy.

Based on photometric data obtained between 1935 and 2017, O − C diagrams were built for 22 RR Lyrae stars in the globular cluster NGC 6171, leading to the discovery of secular period changes in four variables for which we have calculated period change rates . In contrast, we find that 82% of the sample stars have stable periods over the last 82 years. For the stable-period stars, the whole database has been employed to refine their periods. Among the period-changing stars, three (V10, V12, and V16) have decreasing periods that are larger than expected from stellar evolution. Despite these individual cases of significant period change rate, the global average of the measured period changes in the cluster is basically zero, in consonance with theoretical predictions for clusters with redder horizontal branches. The hitherto unpublished observations, now brought into public domain, are employed to calculate a set of times of maximum light, which is used in the present analysis.

We present a CCD VI photometric study of the globular cluster M14. Particular attention is given to the variable stars. This allowed new classifications and cluster membership considerations. New variables are reported; 3 RRc, 18 SR and 1 SX Phe. The Fourier decomposition of RR Lyrae light curves lead to the mean cluster metallicity of [Fe/H]ZW = −1.3 ± 0.2. Several independent methods yield a mean distance of 9.36 ± 0.16 kpc. A Colour-Magnitude diagram outlined by the cluster members enabled a matching with theoretical predictions of isochrones and zero-age horizontal branches, whose fitting to the observations is in good agreement with the above distance and metallicity. The Oosterhoff type of M14 is confirmed as Oo-int, and the pulsating mode distribution of RR Lyrae stars on the HB shows that the bimodal region of the instability strip is shared by RRab and RRc stars. By modelling the mass loss at the RGB after the He flash events, we were able to represent the blue tail of the HB, using a core mass of 0.48 M⊙ and total masses of 0.52-0.55 M⊙. A progenitor star on the MS of 0.84 M⊙ reaches the HB in about 12.5 Gyrs, consonant with previous age determinations of the cluster. Type II Cepheids of M14 may be interpreted as products of post-HB evolution, driven by the complex processes involving the burning of the very thin low mass hydrogen and helium shells of these stars and their minuscule envelopes. No evidences were found in favor of M14 being of extragalactic origin.