Related papers: RR Lyrae Period-Amplitude Diagrams: From Bailey to…

NGC 6388 and NGC 6441 are anomalies among Galactic globular clusters in that they cannot be readily place into either Oosterhoff group I or Oosterhoff group II despite their significant numbers of RR Lyrae variables. The mean pulsation…

In this paper we revisit observational data concerning RR Lyrae stars in Galactic globular clusters, presenting frequency histograms of fundamentalized periods for the 32 clusters having more than 12 pulsators with well recognized period…

We collected a large data set of field RR Lyrae stars (RRLs) by using catalogues already available in the literature and Gaia DR2. We estimated the iron abundances for a sub-sample of 2,382 fundamental RRLs ($\Delta$S method: CaIIK,…

We compare theory and observations for fundamental RR Lyrae in the solar neighborhood and in both Oosterhoff type I (OoI) and type II (OoII) Galactic globular clusters (GGCs). The distribution of cluster RR_ab in the PA_V plane depends not…

We use 3653 (2661 RRab, 992 RRc) RR Lyrae stars (RRLs) with 7D (3D position, 3D velocity, and metallicity) information selected from SDSS, LAMOST, and Gaia EDR3, and divide the sample into two Oosterhoff groups (Oo I and Oo II) according to…

The Galactic Oosterhoff dichotomy between Galactic globular clusters of type I (OoI) and type II (OoII) is often characterized by a difference in the period-amplitude (PA) relations displayed by RR Lyrae ab stars in each type of cluster.…

Although more than eight decades have passed since P. Th. Oosterhoff drew attention to differences in the properties of RR Lyrae variables in globular clusters, the origin and significance of the Oosterhoff groups remain unclear.…

We have undertaken a search for variable stars in the metal-rich globular cluster NGC 6441 using time-series BV photometry. The total number of variables found near NGC 6441 has been increased to ~104, with 48 new variables being found in…

We present an analysis of the period-$V$-amplitude plane for RR0 Lyrae stars (fundamental mode pulsators) with "normal" light curves in the bulge using the MACHO bulge fields. Although bulge globular clusters have RR Lyraes that divide into…

We report on the discovery of a large number of RR Lyrae variable stars in the moderately metal-rich Galactic globular cluster M62 (NGC 6266), which places it among the top three most RR Lyrae-rich globular clusters known. Likely members of…

We have analysed 1455 fundamental mode RR Lyrae stars of the Galactic field, using the All Sky Automated Survey (ASAS) data. The sample covers 75% of the sky and contains objects in the close neighbourhood of the Sun, within 4 kpc distance.…

Low resolution spectra have been used to measure individual metal abundances of RR Lyrae stars in NGC 6441, a Galactic globular cluster known to have very unusual horizontal branch morphology and periods of the RR Lyrae stars for its high…

We discuss the Oosterhoff classification of the unusual, metal-rich globular clusters NGC 6388 and NGC 6441, on the basis of new evolutionary models computed for a range of metallicities. Our results confirm the difficulty in unambiguously…

I point out that the Oosterhoff dichotomy for globular cluster and field RR Lyrae (RRL) stars may place the strongest constraints so far on the number of dwarf spheroidal-like protogalactic fragments that may have contributed to the…

An examination of the period-V amplitude relation for RRab stars (fundamental mode pulsators) with `normal' light curves in the Oosterhoff type I clusters M3 and M107 and in the Oosterhoff type II clusters M9 and M68 reveals that the V…

We present period change rates (dP/dt) for 42 RR Lyrae variables in the globular cluster IC$\,$4499. Despite clear evidence of these period increases or decreases, the observed period change rates are an order of magnitude larger than…

We use color-magnitude diagram synthesis together with theoretical relations from non-linear pulsation models to approach the long-standing problem of the Oosterhoff dichotomy related to the distribution of the mean periods of fundamental…

An analysis of available Galactic globular cluster data on metallicity, ages, the distribution of stars on the horizontal branch and the distribution of the periods of RR0 Lyr variables showed that: 1. The latter two characteristics can be…

We have undertaken a search for variable stars in the metal-rich globular cluster NGC 6388 using time-series BV photometry. Twenty-eight new variables were found in this survey, increasing the total number of variables found near NGC 6388…

We present the first evidence that Oosterhoff type II globular clusters exist in the Andromeda galaxy (M31). On the basis of time-series photometry of the moderately metal-poor ([Fe/H]\sim -1.6 dex) M31 globular cluster G11, obtained with…