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Context. We have obtained BVRI photometry for 21 stellar fields located in directions towards the central $3.5^\circ$ of the Galaxy. Each field is $10\times 10$ arcmin in size, and a total of ~$2 \times 10^5$ stars were measured. Aims. Previous analyses based on 2 Micron All Sky Survey (2MASS) data have shown that these directions have a relatively low extinction ($A_K \simeq 0.25{-}0.30$). The combined optical-infrared sample contains about $3.5 \times 10^4$ stars. This large dataset provides a more detailed extinction map within these surveyed areas than previously derived. The large number of colour indexes available also yielded estimates of effective temperatures and metallicities for the inner bulge stars. Methods. Extinction and effective temperature were estimated for each star based on comparing the observed colours to model predictions. The extinction-corrected colour magnitude diagrams indicate the presence of a broad red giant branch of inner bulge stars detached from the helium-burning red clump of both bulge and disc stars. The red giant stars belonging to the bulge form a wide sequence mostly due to variable metallicity. As previously done by other authors, we derived photometric metallicities by comparing the position of each bulge red giant to a set of templates based on Galactic globular clusters. Results. We confirm the existence of regions with $A_K < 0.30$ in these fields, as attested by analysis of the stellar colours. The resulting metallicity distribution function is peaked at $\rm [Fe/H] \simeq -0.25$ and extends to supersolar values up to $\rm [Fe/H] \simeq 0.3$. This distribution is similar among the 4 regions, with relatively small field-to-field variations within the central 1kpc. Our results are also in good agreement with the metallicity distributions from other authors, both photometric and spectroscopic. Put together, this result strongly limits the existence of an abundance gradient in the inner bulge.

Multiband photometry towards the inner Galactic bulge: extinction, atmospheric parameters, and metallicities