Gaia DR2 was released in April 2018 and contains a photometric catalogue of more than 1 billion sources. This release contains colour information in the form of integrated BP and RP photometry in addition to the latest G-band photometry. The level of uncertainty can be as good as 2 mmag with some residual systematics at the 10 mmag level. The addition of colour information greatly enhances the value of the photometric data for the scientific community. A high level overview of the photometric processing, with a focus on the improvements with respect to Gaia DR1, was given. The definition of the Gaia photometric system, a crucial part of the calibration of the photometry, was also explained. Finally, some of the photometric improvements expected for the next data release were described.

The Gaia spectro-photometric data consist of white-light G magnitudes from unfiltered fluxes measured in the astrometric field CCDs, and low resolution prism spectra measured in the blue (BP) and red (RP) CCDs. The integrated flux of these BP and RP spectra will yield GBP and GRP magnitudes as two broad passbands. The internal flux calibration will correct for all possible instrumental effects and produce mean spectral energy distributions in an internally defined flux scale. The purpose of the absolute flux calibration is to tie this flux scale to physical units by means of spectro-photometric standard stars that rely ultimately on Vega’s absolute flux scale. The characteristics of the Gaia spectrophotometric system will be described, and the absolute calibration process will be outlined and compared with examples of classical systems.

Heliocentric (UVW) and galactocentric (VR VΦ VZ) space motions were derived for 38 RR Lyrae (RRL) and 79 blue horizontal branch (BHB) stars in a 200-sq degree area near the North Galactic Pole (NGP). A kinematic analysis of the 26 RRL and 52 BHB stars whose height (Z) above the plane is < 8 kpc shows that the sample is not homogeneous. Our BHB sample shows zero galactic rotation and roughly isotropic velocity dispersions. whereas the RRL sample shows a definite retrograde rotation and non-isotropic velocity dispersions. The combined BHB and RRL sample shows a smaller retrograde rotation that is similar to that found by Majewski et al. (1996) for a sample of subdwarfs in SA 57 at the NGP. There are significantly more RRL with negative W-velocity (streaming down) than positive W-velocity, whereas the numbers of BHB stars are comparable. This indicates the presence near the NGP of an accreted halo component that is rich in RRL (probably Oosterhoff type I) stars.

These results are presented in detail in a forthcoming paper (Kinman et al. 2007).

New GSC-II proper motions of RR Lyrae and Blue Horizontal Branch (BHB) stars near the North Galactic Pole are used to show that the Galactic Halo 5 kpc above the Plane has a significantly retrograde galactic rotation.

The study of the oldest stellar populations in the Galaxy provides important cosmological information (e.g. a lower limit to the age of the Universe) and helps constraining models of galaxy formation. The most commonly used stellar age indicators are reviewed and discussed, and the impact of GAIA on these topics is estimated.

We report on new B, V and I CCD photometry of the globular cluster M54 that was aimed at the study of its variable stars. With respect to the previous most recent work on M54 we have nearly doubled the number of detected variable stars: M54 can now be classified as intermediate in the Oosterhoff groups. The metallicity can be estimated for the cluster and field red giant stellar population, and for the variables.

Candidate RR Lyrae variables have been identified for the first time in four globular clusters of the Andromeda galaxy (M31). Using HST-WFPC2 archive observations we have found 2, 4, 11 and 8 RR Lyrae variables of both ab and c Bailey types in G11, G33, G64 and G322 (Sargent et al., 1977), respectively. These numbers are quite consistent with the horizontal branch morphology exhibited by the four clusters.

We report new, as yet unpublished HST/WFPC2 V, I photometry of 9 globular clusters in M31. These are part of a total sample of 19 clusters on M31 with well observed CMDs from HST. The clusters have the full range of horizontal branch morphologies from blue HB to red clump, but none are found with an extreme (blue only) horizontal branch. Plots of HB type vs [Fe/H] are similar to those of Galactic clusters, including a hint of second parameter clusters. Sixteen halo fields adjacent to the observed clusters included in our images have been analysed. The M31 halo metallicity distribution peaks at [Fe/H]=-0.7 dex with a tail toward low metallicity, resembling the halo of NGC 5128; metal-rich giants appear to be centrally concentrated.

For Commission 27 these triennial reports have traditionally been very compact literature reviews of all the fields relevant to our commission. For several triennia we have been discussing the relevance of them, and asking just who their readership is. It seems that only a few people read them, and fewer use them as introductions to the subject – supposedly one of their prime purposes. The major beneficiaries have been the writers, who have been forced to do three-year reviews of their subjects. The IAU EC gave us the option this triennium of a shorter, four-page report to which the majority of the SOC agreed.

The mass of RR Lyrae variables has been a controversial problem for about a decade: while the stellar evolution theory predicts masses ranging between 0.65 and 0.75 M⊙ for Oosterhoff type I and II clusters respectively, the stellar pulsation theory predicts smaller masses (0.55 and 0.65 respectively) using the double-mode pulsators. Simon (1990, M.N.R.A.S. 246, 70), comparing hydrodynamical models with observed stars by means of Fourier parameters, has found relations between the stellar mass and its luminosity, pulsation period, Helium content and Fourier parameter ϕ31. Combining his equations we obtain:

(1)

from which one can estimate the stellar mass by using the observable quantities P and ϕ31 and a luminosity scale, e.g. the one derived by Cacciari, Clementini and Fernley (1992, Astrophys. J. in press).

New CCD BVI observations of RR Lyrae variables in M3 are presented. Mean magnitudes and colours are derived, as well as their relations with periods and amplitudes, and comparisons are made with previous data (Sandage 1981, 1990). Preliminary results are presented on the temperature distribution of the variables and the period-shift effect with respect to M15 and M68.

We have applied two slightly different formulations of the Baade-Wesselink (B-W) method: (i) the Infrared Flux (IF) method (Fernley et al 1990a), and (ii) the Surface Brightness (SB) method (Cacciari et al 1989a), to derive the absolute magnitude of the field RR Lyrae stars UU Cet, RV Phe and W Tuc. The photometric (BVRIJHK) and radial velocity data used in the analysis have been published in Cacciari et al (1987, 1989b, 1991) and Clementini et al (1990). The Walraven photometry used in the IF method is by Lub (1977). Typical accuracies for individual data points are ±0.01-0.02 mag for the magnitudes and colours, and ±1-2km/sec for the radial velocity data. Excluding RV Phe, which may be affected by Blazhko effects, the results obtained from the IF and SB methods are rather consistent especially when the V–K colour is used in the SB method. For details see Cacciari et al. (1991).

The surface brightness version of the Baade-Wesselink method, has been applied to 7 field RR Lyrae stars with metallicity ranging from [Fe/H]= −0.2 to −1.5. V magnitudes, V-R and V-I colors and CORAVEL radial velocities were used, and the analysis was performed over a restricted phase range in order to avoid the complications caused by the pulsating atmospheres. The combination with previous results of the B-W method, which used comparable criteria (Jones, Carney, & Latham, 1988, preprint; Jameson, Fernley, & Longmore 1987, in press M.N.R.A.S; Cohen & Gordon 1987, Ap.J., 318, 215) leads to the following relation between the absolute luminosity and metallicity:

Mv = (1.0 ± 0.05) + (0.17 ± 0.05) [Fe/H]

This relation is in very good agreement with the preliminary results found by Liu and Janes (this volume). The application of the above relation to the RR Lyraes in M31 and the Magellanic Clouds leads to distance moduli of (m–M)o= 24.21 ± 0.20 for M31, (m–M)o = 18.26 ± 0.20 for the LMC, and (m–M)o =18.85 ± 0.20 for the SMC, and the distance to the galactic center turns out to be approximately 7.2 kpc. From the absolute magnitude of the RR Lyraes and adopting a constant visual magnitude difference between the RR Lyraes and the turn-off ΔV = 3.55 (Buonanno 1986, Mem.S.A.It., 57, 333), we estimate ages of 18.8 and 15.7 Gyr for globular clusters of metallicity [Fe/H] = −2.2 (e.g. M92) and [Fe/H] = −0.8 (e.g. 47 Tuc) respectively, using the age-turnoff luminosity relation derived by Sandage (1982, Ap.J., 252, 553) or 20.9 and 16.9 Gyr using Buonanno’s relation.

We have taken UBVRI photoelectric photometry and CORAVEL radial velocities for 6 field RR Lyraes, i.e. SW And, YZ Cap, SW Dra, SS For, RV Phe and V440 Sgr, with the purpose of applying the Baade-Wesselink (B-W) method and determining their absolute magnitudes. The present improvements with respect to previous applications of the method are: a) the use of the (V-I) color, which shows the smallest sensitivity to gravity and metal abundance in the color-visual surface brightness plane. b) the use of a new grid of model atmospheres by Buser and Kurucz (1986), which include an improved treatment of opacity and convection. They provide a complete set of models for the relevant values of effective temperatures, gravities and metal abundances, thus avoiding the need of inaccurate interpolations or extrapolations.

New BV photographic photometry for 56 RR Lyrae variables in M15, combined with original measures by Sandage, Katem and Sandage (1981), yield data of improved precision which enable a rediscussion of the RR Lyrae characteristics and Oosterhoff problem.

The flux originating from the central stars of 27 planetary nebulae in the spectral range 1200-2000 Å has been deduced from the analysis of a large number of released IUE low resolution spectra.

The assumption that individual globular clusters formed from chemically homogeneous protosystems, which seems to be valid for some clusters, is clearly not valid in others, e.g., ω Cen and M22 (see Freeman and Rodgers 1975, Hesser, Hartwick and McClure 1976, Butler, Dickens and Epps 1978 and references quoted therein). One indicator of chemical inhomogeneity is the width of the cluster giant branch in the C-M diagram, because the B-V colors for the cool giants are sensitive to heavy element abundance. NGC 3201 and M 5 show narrow giant branches (Lee 1977, Arp 1962) and so are not expected to show large star-to-star heavy element differences.