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We provide the largest and most homogeneous sample of $\alpha$-element (Mg, Ca, Ti) and iron abundances for field RR Lyrae (RRLs, 162 variables) by using high-resolution spectra. The current measurements were complemented with similar abundances available in the literature for 46 field RRLs brought to our metallicity scale. We ended up with a sample of old (t$\ge$ 10 Gyr), low-mass stellar tracers (208 RRLs: 169 fundamental, 38 first overtone, 1 mixed mode) covering three dex in iron abundance (-3.00$\le$[Fe/H]$\le$0.24). We found that field RRLs are $\sim$0.3 dex more $\alpha$-poor than typical Halo tracers in the metal-rich regime, ([Fe/H]$\ge$-1.2) while in the metal-poor regime ([Fe/H]$\le$-2.2) they seem to be on average $\sim$0.1 dex more $\alpha$-enhanced. This is the first time that the depletion in $\alpha$-elements for solar iron abundances is detected on the basis of a large, homogeneous and coeval sample of old stellar tracers. Interestingly, we also detected a close similarity in the [$\alpha$/Fe] trend between $\alpha$-poor, metal-rich RRLs and red giants (RGs) in the Sagittarius dwarf galaxy as well as between $\alpha$-enhanced, metal-poor RRLs and RGs in ultra faint dwarf galaxies. These results are supported by similar elemental abundances for 46 field Horizontal Branch (HB) stars. These stars share with RRLs the same evolutionary phase and the same progenitors. This evidence further supports the key role that old stellar tracers play in constraining the early chemical enrichment of the Halo and, in particular, in investigating the impact that dwarf galaxies have had in the mass assembly of the Galaxy.

Accurate metallicities of RR Lyrae are extremely important in constraining period-luminosity-metallicity relationships (PLZ), particularly in the near-infrared. We analyse 69 high-resolution spectra of Galactic RR Lyrae stars from the Southern African Large Telescope (SALT). We measure metallicities of 58 of these RR Lyrae stars with typical uncertainties of 0.13 dex. All but one RR Lyrae in this sample has accurate ({\sigma}_parallax ~ 10%) parallax from Gaia. Combining these new high resolution spectroscopic abundances with similar determinations from the literature for 93 stars, we present new PLZ relationships in WISE W1 and W2 magnitudes, and the Wesenheit magnitudes W(W1,V-W1) and W(W2,V-W2)., Comment: 16 pages, 13 figures, accepted in MNRAS

We collected over 6000 high-resolution spectra of four dozen field RR Lyrae (RRL) variables pulsating either in the fundamental (39 RRab) or in the first overtone (9 RRc) mode. We measured radial velocities (RVs) of four strong metallic and four Balmer lines along the entire pulsational cycle and derived RV amplitudes with accuracies better than 1$-$2~\kmsec. The new amplitudes were combined with literature data for 23~RRab and 3~RRc stars (total sample 74 RRLs) which allowed us to investigate the variation of the Bailey diagram (photometric amplitude versus period) when moving from optical to mid-infrared bands and to re-cast the Bailey diagram in terms of RV amplitudes. We found that RV amplitudes for RRab are minimally affected by nonlinear phenomena (shocks) and multi-periodicity (Blazhko effect). The RV slope ($\log P$--A(V$_r$)) when compared with the visual slope ($\log P$--A($V$)) is shallower and the dispersion, at fixed period, decreases by a factor of two. We constructed homogeneous sets of Horizontal Branch evolutionary models and nonlinear, convective pulsation models of RRLs to constrain the impact of evolutionary effects on their pulsation properties. Evolution causes, on the Bailey diagram based on RV amplitudes, a modest variation in pulsation period and a large dispersion in amplitude. The broad dispersion in period of the Bailey diagram is mainly caused by variation in RRL intrinsic parameters (stellar mass, chemical composition). Empirical evidence indicates that RV amplitudes are an optimal diagnostic for tracing the mean effective temperature across the RRab instability strip., 14 pages, 4 figures, 1 table, Accepted on ApJ Letter

We present homogeneous and accurate iron abundances for almost four dozen (47) of Galactic Cepheids using high-spectral resolution (R$\sim$40,000) high signal-to-noise ratio (S/N $\ge$ 100) optical spectra collected with UVES at VLT. A significant fraction of the sample (32) is located in the inner disk (RG $le$ 6.9 kpc) and for half of them we provide new iron abundances. Current findings indicate a steady increase in iron abundance when approaching the innermost regions of the thin disk. The metallicity is super-solar and ranges from 0.2 dex for RG $\sim$ 6.5 kpc to 0.4 dex for RG $\sim$ 5.5 kpc. Moreover, we do not find evidence of correlation between iron abundance and distance from the Galactic plane. We collected similar data available in the literature and ended up with a sample of 420 Cepheids. Current data suggest that the mean metallicity and the metallicity dispersion in the four quadrants of the Galactic disk attain similar values. The first-second quadrants show a more extended metal-poor tail, while the third-fourth quadrants show a more extended metal-rich tail, but the bulk of the sample is at solar iron abundance. Finally, we found a significant difference between the iron abundance of Cepheids located close to the edge of the inner disk ([Fe/H]$\sim$0.4) and young stars located either along the Galactic bar or in the nuclear bulge ([Fe/H]$\sim$0). Thus suggesting that the above regions have had different chemical enrichment histories. The same outcome applies to the metallicity gradient of the Galactic bulge, since mounting empirical evidence indicates that the mean metallicity increases when moving from the outer to the inner bulge regions., Comment: 10 pages, 5 figures; Corrected typos, corrected Table 2

International audience; Context. The CoRoT - Convection Rotation and planetary Transits - space mission is a great opportunity for monitoring stars with excellent time-sampling and unprecedented photometric precision for up to 150 days. As an important benefit, high-quality RR Lyrae light curves are obtained with a quasi-uninterrupted coverage over several pulsation and Blazhko cycles. Aims: The Blazhko effect in RR Lyrae stars is an unsolved problem of astrophysics. We used the high-precision space data to contribute more precise knowledge to explain the possible physical processes behind the phenomenon. Methods: We applied different period-finding techniques including Period04, MuFrAn, PDM and SigSpec. Amplitude and phase modulation were investigated by an analytical function method as well as with the traditional O-C diagrams. Results: The Blazhko modulation frequency is directly detected in the spectrum, as well as its first and second harmonics. It shows the non-linear nature of the Blazhko modulation. Besides the triplets, further higher-order modulation side peaks appear around the pulsation frequency as quintuplet, septuplet, nonuplet, undecaplet, tredecaplet, quindecaplet and sepdecaplet structures. Additional frequencies, not belonging to the classical multiplet structures, are detected, as well as their linear combinations with the fundamental radial mode. We interpret these additional terms as non-radial modes. During the five consecutive Blazhko cycles, there is a shift of the maximum phase around 0.011 pulsation phase which is likely the consequence of a long term modulation.

In a previous paper, we reported on the discovery of more than a hundred new Cepheid variables in the Sculptor Group spiral NGC 300 from wide-field images taken in the B and V photometric bands at ESO/La Silla. In this paper, we present additional VI data, derive improved periods and mean magnitudes for the variables, and construct period-luminosity relations in the V, I and the reddening-independent (V-I) Wesenheit bands using 58 Cepheid variables with periods between 11 and 90 days. We obtain tightly defined relations, and by fitting the slopes determined for the LMC Cepheids by the OGLE II Project we obtain reddening-corrected distances to the galaxy in all bands. We adopt as our best value the distance derived from the reddening-free Wesenheit magnitudes, which is 26.43 $\pm$ 0.04 (random) $\pm$ 0.05 (systematic) mag. We argue that our current distance result for NGC 300 is the most accurate which has so far been obtained using Cepheid variables, and that it is largely free from systematic effects due to metallicity, blending, and sample selection. It agrees very well with the recent distance determination from the tip of the red giant branch method obtained from HST data by Butler et al. (2004), and it is consistent with the Cepheid distance to NGC 300 which was derived by Freedman et al. (2001) from CCD photometry of a smaller sample of stars., Comment: Latex, Astronomical Journal in press

We present new theoretical Period-Radius (PR) relations for first overtone Galactic Cepheids. Current predictions are based on several sequences of nonlinear, convective pulsation models at solar chemical composition (Y=0.28, Z=0.02) and stellar masses ranging from 3.0 to 5.5 Mo. The comparison between predicted and empirical radii of four short-period Galactic Cepheids suggests that QZ Nor and EV Sct are pulsating in the fundamental mode, whereas Polaris and SZ Tau pulsate in the first overtone. This finding supports the mode identifications that rely on the comparison between direct and Period-Luminosity (PL) based distance determinations but it is somewhat at variance with the mode identification based on Fourier parameters. In fact, we find from our models that fundamental and first overtone pulsators attain, for periods ranging from 2.7 to 4 d, quite similar phi_21 values, making mode discrimination from this parameter difficult. The present mode identifications for our sample of Cepheids are strengthened by the accuracy of their empirical radius estimates, as well as by the evidence that predicted fundamental and first overtone radii do not show, within the current uncertainty on the Mass-Luminosity (ML) relation, any degeneracy in the same period range. Accurate radius determinations are therefore an excellent tool to unambiguously determine the pulsation modes of short-period Cepheids., 15 pages, 3 postscript figures, accepted for publication on ApJ Letters