Abstract. We carried out a period analysis on ∼3700 RR Lyrae stars on two Schmidt fields centred on (l, b) = (3.1 • , −7.1 • ) and (6.6 • , −10.8 • ) respectively, covering an area of ∼50 deg 2 . These stars are distributed almost evenly between the Sagittarius dwarf galaxy (Sgr) and the Milky Way. For Sgr members, the average periods are P ab = 0.574 d and Pc = 0.322 d for RRab and RRc stars respectively. This places Sgr in the long-period tail of the Oosterhoff I group. We report the detection of 53 double-mode RR Lyrae stars (RRd) within our sample. The magnitude of 40 of these stars is consistent with membership in Sgr whereas 13 RRds are located within our Galaxy. We also found 13 RR Lyraes (5 in Sgr and 8 in the Galaxy) exhibiting two closely spaced frequencies, most probably related to non-radial pulsations. The period distribution of the RR Lyrae variables in Sgr is compared to those of other Milky Way satellites. We find a remarkable similarity between the RR Lyrae populations in Sgr and the Large Magellanic Cloud (LMC), suggesting that these galaxies have similar horizontal branch morphologies. This may indicate that Sgr and the LMC started their formation under similar conditions. Using various photometric indicators, we estimate the metallicity of the RR Lyrae stars in Sgr and find [Fe/H] −1.6 dex with a dispersion of ∼±0.5 dex around this value and a minor but significant population at .−2.0 dex. We do not find evidence for a spatial metallicity gradient in the RR Lyrae population of Sgr. From the spatial distribution of RR Lyraes, we find that the main body of Sgr contains ∼4200 RRab stars. Assuming that population gradients are negligible in Sgr, we find MV (Sgr) −13.9 +0.4 −0.6 mag for the main body. If Sgr has been stripped of 50% of its mass through Galactic tides, as assumed by some models, it would imply a total absolute magnitude of ∼−14.7 mag for this galaxy. Such a luminosity would be consistent with the empirical metallicity/luminosity relation for dwarf spheroidal galaxies .

We present results of a microlensing survey toward the Andromeda Galaxy (M 31) carried out during four observing seasons at the Isaac Newton Telescope (INT). This survey is part of the larger microlensing survey toward M 31 performed by the Microlensing Exploration of the Galaxy and Andromeda (MEGA) collaboration. Using a fully automated search algorithm, we identify 14 candidate microlensing events, three of which are reported here for the first time. Observations obtained at the Mayall telescope are combined with the INT data to produce composite lightcurves for these candidates. The results from the survey are compared with theoretical predictions for the number and distribution of events. These predictions are based on a Monte Carlo calculation of the detection efficiency and disk-bulge-halo models for M 31. The models provide the full phase-space distribution functions (DFs) for the lens and source populations and are motivated by dynamical and observational considerations. They include differential extinction and span a wide range of parameter space characterised primarily by the mass-to-light ratios for the disk and bulge. For most models, the observed event rate is consistent with the rate predicted for self-lensing -a MACHO halo fraction of 30% or higher can be ruled at the 95% confidence level. The event distribution does show a large near-far asymmetry hinting at a halo contribution to the microlensing signal. Two candidate events are located at particularly large projected radii on the far side of the disk. These events are difficult to explain by self lensing and only somewhat easier to explain by MACHO lensing. A possibility is that one of these is due to a lens in a giant stellar stream.

Abstract. We present the first M 31 candidate microlensing events from the Microlensing Exploration of the Galaxy and Andromeda (MEGA) survey. MEGA uses several telescopes to detect microlensing towards the nearby Andromeda galaxy, M 31, in order to establish whether massive compact objects are a significant contribution to the mass budget of the dark halo of M 31. The results presented here are based on observations with the Isaac Newton Telescope on La Palma, during the 1999/00 and 2000/01 observing seasons. In this data set, 14 variable sources consistent with microlensing have been detected, 12 of which are new and 2 have been reported previously by the POINT-AGAPE group. A preliminary analysis of the spatial and timescale distributions of the candidate events support their microlensing nature. We compare the spatial distributions of the candidate events and of long-period variable stars, assuming the chances of finding a long-period variable and a microlensing event are comparable. The spatial distribution of our candidate microlensing events is more far/near side asymmetric than expected from the detected long-period variable distribution. The current analysis is preliminary and the asymmetry not highly significant, but the spatial distribution of candidate microlenses is suggestive of the presence of a microlensing halo.

Abstract. We estimate the optical depth, time-scale distribution and fraction of microlensing events originating from sources in the Sagittarius dwarf galaxy (Sgr) lensed by deflectors in the Milky Way. These events have a time-scale longer by a factor ∼1.3 than the MW/MW events and occur mainly on sources fainter than V ∼ 21 mag below Sgr's turn off. The fraction of events involving a source in Sgr depends on the location and extinction of the field and on the limiting magnitude of the survey. The contribution of the MW/Sgr events is negligible ( < ∼ 1%) at very low latitudes (|b| < ∼ 2 • ) but increases continuously towards higher |b| and becomes dominant near the highest density region of the dwarf galaxy. Sgr is present within the fields of current microlensing surveys and any optical depth map inferred from observations will become biased by the presence of Sgr towards higher |b| where the contribution of MW/Sgr events is significant. Systematic spectroscopic measurements on the sources of all the microlensing events may allow detection of this kind of event for which the degeneracy on the lens mass can be significantly reduced.

We investigate HST/ACS and WFPC2 images at the positions of five candidate microlensing events from a large survey of variability in M31 (MEGA). Three closely match unresolved sources, and two produce only flux upper limits. All are confined to regions of the color-magnitude diagram where stellar variability is unlikely to be easily confused with microlensing. Red variable stars cannot explain these events (although background supernovae are possible for two). If these lenses arise in M31's halo, they are due to masses 0.15 ! m/M ! , (95% certainty, for a d-function mass distribution), with brown dwarfs for disk lenses and stellar masses 0.49 for bulge lenses.