[en] We present the first time-series study of the ultra-faint dwarf galaxy Hercules. Using a variety of telescope/instrument facilities we secured about 50 V and 80 B epochs. These data allowed us to detect and characterize 10 pulsating variable stars in Hercules. Our final sample includes six fundamental-mode (ab-type) and three first-overtone (c-type) RR Lyrae stars, and one Anomalous Cepheid. The average period of the ab-type RR Lyrae stars, (P_ab) = 0.68 days (σ = 0.03 days), places Hercules in the Oosterhoff II group, as found for almost the totality of the ultra-faint dwarf galaxies investigated so far for variability. The RR Lyrae stars were used to obtain independent estimates of the metallicity, reddening, and distance to Hercules, for which we find [Fe/H] = –2.30 ± 0.15 dex, E(B – V) = 0.09 ± 0.02 mag, and (m – M)₀ = 20.6 ± 0.1 mag, in good agreement with the literature values. We have obtained a V, B – V color-magnitude diagram (CMD) of Hercules that reaches V ∼ 25 mag and extends beyond the galaxy's half-light radius over a total area of 40' × 36'. The CMD and the RR Lyrae stars indicate the presence of a population as old and metal-poor as (at least) the Galactic globular cluster M68.

[en] We present a B, V color-magnitude diagram (CMD) of the Milky Way dwarf satellite Ursa Major II (UMa II), spanning the magnitude range from V ∼ 15 to V ∼ 23.5 mag and extending over an 18 × 18 arcmin² area centered on the Galaxy. Our photometry goes down to about 2 mag below the Galaxy's main-sequence turnoff that we detected at V ∼ 21.5 mag. We have discovered a bona fide RR Lyrae variable star in UMa II, which we use to estimate a conservative dereddened distance modulus for the galaxy of (m – M)₀ = 17.70 ± 0.04 ± 0.12 mag, where the first error accounts for the uncertainties of the calibrated photometry, and the second reflects our lack of information on the metallicity of the star. The corresponding distance to UMa II is 34.7^+0.6_–0.7(^+2.0_–1.9) kpc. Our photometry shows evidence of a spread in the Galaxy's subgiant branch, compatible with a spread in metal abundance in the range between Z = 0.0001 and Z = 0.001. Based on our estimate of the distance, a comparison of the fiducial lines of the Galactic globular clusters M68 and M5 ([Fe/H] = –2.27 ± 0.04 dex and –1.33 ± 0.02 dex, respectively), with the position on the CMD of spectroscopically confirmed Galaxy members, may suggest the existence of stellar populations of different metal abundance/age in the central region of UMa II.

[en] On the basis of recently computed nonlinear convective pulsation models of Galactic Cepheids, spanning wide ranges of input stellar parameters, we derive theoretical mass-dependent Period–Wesenheit relations in the Gaia bands, namely, G, G _BP, and G _BR, that are found to be almost independent of the assumed efficiency of superadiabatic convection. The application to a selected subsample of the Gaia Data Release 2 Galactic Cepheids database allows us to derive mass-dependent estimates of their individual distances. By imposing their match with the astrometric values inferred from Gaia, we are able to evaluate the individual mass of each pulsator. The inferred mass distribution is peaked around 5.6M _⊙ and 5.4M _⊙ for the F and FO pulsators, respectively. If the estimated Gaia parallax offset $⟨<!--\; ???\; -->\mathrm{\Delta <!--\; ??\; -->}\mathrm{\varpi <!--\; ??\; -->}⟩<!--\; ???\; -->=0.046$ mas is applied to Gaia parallaxes before imposing their coincidence with the theoretical ones, the inferred mass distribution is found to shift toward lower masses, namely, ∼5.2M _⊙ and 5.1M _⊙ for the F and FO pulsators, respectively. The comparison with independent evaluations of the stellar masses, for a subset of binary Cepheids in our sample, seems to support the predictive capability of the current theoretical scenario. By forcing the coincidence of our mass determinations with these literature values we derive an independent estimate of the mean offset to be applied to Gaia DR2 parallaxes, $⟨<!--\; ???\; -->\mathrm{\Delta <!--\; ??\; -->}\mathrm{\varpi <!--\; ??\; -->}⟩<!--\; ???\; -->=0.053\pm <!--\; ??\; -->0.029$ mas, slightly higher but in agreement within the errors with the Riess et al. value.

[en] We present B, V, I time-series photometry of the Coma Berenices dwarf spheroidal galaxy, a faint Milky Way (MW) satellite, recently discovered by the Sloan Digital Sky Survey. We have obtained V, B - V and V, V - I color-magnitude diagrams that reach V ∼ 23.0-23.2 mag showing the galaxy turnoff at V ∼ 21.7 mag, and have performed the first study of the variable star population of this new MW companion. Two RR Lyrae stars (a fundamental-mode, RRab, and a first overtone, RRc, pulsator) and a short period variable with period P = 0.12468 days were identified in the galaxy. The RRab star has a rather long period of P _ab = 0.66971 days and is about 0.2 mag brighter than the RRc variable and other nonvariable stars on the galaxy horizontal branch (HB). In the period-amplitude diagram, the RRab variable falls closer to the loci of Oosterhoff type-II systems and evolved fundamental-mode RR Lyrae stars in the Galactic globular cluster M3. The average apparent magnitude of the galaxy HB, (V _HB) = 18.64 ± 0.04 mag, leads to a distance modulus for the Coma dSph μ₀ = 18.13 ± 0.08 mag, corresponding to a distance d = 42⁺²_-1 kpc, by adopting a reddening E(B - V) = 0.045 ± 0.015 mag and a metallicity [Fe/H] =-2.53 ± 0.05 dex.

[en] In this paper, the synthetic period-luminosity (P-L) relations in Spitzer's IRAC bands, based on a series of theoretical pulsation models with varying metal and helium abundance, were investigated. Selected sets of these synthetic P-L relations were compared to the empirical IRAC band P-L relations recently determined from Galactic and Magellanic Clouds Cepheids. For the Galactic case, synthetic P-L relations from model sets with (Y = 0.26, Z = 0.01), (Y = 0.26, Z = 0.02), and (Y = 0.28, Z = 0.02) agree with the empirical Galactic P-L relations derived from the Hubble Space Telescope parallaxes. For Magellanic Cloud Cepheids, the synthetic P-L relations from model sets with (Y = 0.25, Z = 0.008) agree with both of the empirical Large Magellanic Cloud (LMC) and Small Magellanic Cloud P-L relations. Analysis of the synthetic P-L relations from all model sets suggested that the IRAC band P-L relations may not be independent of metallicity, as the P-L slopes and intercepts could be affected by the metallicity and/or helium abundance. We also derive the synthetic period-color (P-C) relations in the IRAC bands. Non-vanishing synthetic P-C relations were found for certain combinations of IRAC band filters and metallicity. However, the synthetic P-C relations disagreed with the [3.6]-[8.0] P-C relation recently found for the Galactic Cepheids. The synthetic [3.6]-[4.5] P-C slope from the (Y = 0.25, Z = 0.008) model set, on the other hand, is in excellent agreement to the empirical LMC P-C counterpart, if a period range 1.0 < log (P) < 1.8 is adopted.

[en] We present results from the first combined study of variable stars and star formation history (SFH) of the Milky Way 'ultra-faint' dwarf (UFD) galaxy Leo T, based on F606W and F814W multi-epoch archive observations obtained with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. We have detected 14 variable stars in the galaxy. They include one fundamental-mode RR Lyrae star and 11 Anomalous Cepheids with periods shorter than 1 day, thus suggesting the occurrence of multiple star formation episodes in this UFD, of which one about 10 Gyr ago produced the RR Lyrae star. A new estimate of the distance to Leo T of 409⁺²⁹_–27 kpc (distance modulus of 23.06 ± 0.15 mag) was derived from the galaxy's RR Lyrae star. Our V, V – I color-magnitude diagram (CMD) of Leo T reaches V ∼ 29 mag and shows features typical of a galaxy in transition between dwarf irregular and dwarf spheroidal types. A quantitative analysis of the SFH, based on the comparison of the observed V, V – I CMD with the expected distribution of stars for different evolutionary scenarios, confirms that Leo T has a complex SFH dominated by two enhanced periods about 1.5 and 9 Gyr ago, respectively. The distribution of stars and gas shows that the galaxy has a fairly asymmetric structure.

[en] We have performed the first study of the variable star population of Ursa Major I (UMa I), an ultra-faint dwarf satellite recently discovered around the Milky Way (MW) by the Sloan Digital Sky Survey. Combining time series observations in the B and V bands from four different telescopes, we have identified seven RR Lyrae stars in UMa I, of which five are fundamental-mode (RRab) and two are first-overtone pulsators (RRc). Our V, B – V color-magnitude diagram of UMa I reaches V ∼ 23 mag (at a signal-to-noise ratio of ∼6) and shows features typical of a single old stellar population. The mean pulsation period of the RRab stars (P_ab) = 0.628, σ = 0.071 days (or (P_ab) = 0.599, σ = 0.032 days, if V4, the longest period and brightest variable, is discarded) and the position on the period-amplitude diagram suggest an Oosterhoff-intermediate classification for the galaxy. The RR Lyrae stars trace the galaxy horizontal branch (HB) at an average apparent magnitude of (V(RR)) = 20.43 ± 0.02 mag (average on six stars and discarding V4), giving in turn a distance modulus for UMa I of (m – M)₀ = 19.94 ± 0.13 mag, distance d = 97.3^+6.0_-5.7 kpc, in the scale where the distance modulus of the Large Magellanic Cloud is 18.5 ± 0.1 mag. Isodensity contours of UMa I red giants and HB stars (including the RR Lyrae stars identified in this study) show that the galaxy has an S-shaped structure, which is likely caused by the tidal interaction with the MW. Photometric metallicities were derived for six of the UMa I RR Lyrae stars from the parameters of the Fourier decomposition of the V-band light curves, leading to an average metal abundance of [Fe/H] = –2.29 dex (σ = 0.06 dex, average on six stars) on the Carretta et al. metallicity scale.