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We investigate interstellar extinction curve variations towards ~4 deg² of the inner MilkyWay in VIJKs photometry from the OGLE-III (third phase of the Optical Gravitational Lensing Experiment) and VVV (VISTA Variables in the Via Lactea) surveys, with supporting evidence from diffuse interstellar bands and F435W, F625W photometry. We obtain independent measurements towards ~2000 sightlines of AI, E(V - I), E(I - J) and E(J - Ks), with median precision and accuracy of 2 per cent. We find that the variations in the extinction ratios AI/E(V - I), E(I - J)/E(V - I) and E(J - Ks)/E(V - I) are large (exceeding 20 per cent), significant and positively correlated, as expected. However, both the mean values and the trends in these extinction ratios are drastically shifted from the predictions of Cardelli and Fitzpatrick, regardless of how RV is varied. Furthermore, we demonstrate that variations in the shape of the extinction curve have at least two degrees of freedom, and not one (e.g. RV), which we confirm with a principal component analysis. We derive a median value of (AV/AKs) = 13.44, which is ~60 per cent higher than the 'standard' value. We show that theWesenheit magnitude WI = I - 1.61(I - J) is relatively impervious to extinction curve variations. Given that these extinction curves are linchpins of observational cosmology, and that it is generally assumed that RV variations correctly capture variations in the extinction curve, we argue that systematic errors in the distance ladder from studies of Type Ia supernovae and Cepheids may have been underestimated. Moreover, the reddening maps from the Planck experiment are shown to systematically overestimate dust extinction by ~100 per cent and lack sensitivity to extinction curve variations. [ABSTRACT FROM AUTHOR]

We model the split red clump of the Galactic bulge in OGLE-III photometry, and compare the results to predictions from two N-body models. Our analysis yields precise maps of the brightness of the two red clumps, the fraction of stars in the more distant peak, and their combined surface density. We compare the observations to predictions from two N-body models previously used in the literature. Both models correctly predict several features as long as one assumes an angle αBar ≈30° between the Galactic bar's major axis and the line of sight to the Galactic Centre. In particular that the fraction of stars in the faint red clump should decrease with increasing longitude. The biggest discrepancies between models and data are in the rate of decline of the combined surface density of red clump stars towards negative longitudes and of the brightness difference between the two red clumps towards positive longitudes, with neither discrepancy exceeding ~25 per cent in amplitude. Our analysis of the red giant luminosity function also yields an estimate of the red giant branch bump parameters towards these high-latitude fields, and evidence for a high rate (~25 per cent) of disc contamination in the bulge at the colour and magnitude of the red clump, with the disc contamination rate increasing towards sightlines further distant from the plane. [ABSTRACT FROM AUTHOR]