Transformations between the Gaia GBP, G and GRP and the Johnson-Cousins Photometric System1

We used the Gaia GBP, G and GRP magnitudes of a set of Landolt standard stars to obtain a set of transformations to the Johnson-Cousins U, B, V, R and I photometric system. The interval of validity for these transformations is 8 < Mag < 16. The values of the residuals are smaller than ∼1.0 for U, smaller than ∼0.1 for B and smaller than ∼0.05 for V, R and I. We compare our transformations with those of Riello and the transformed values we calculate are very similar, where the residuals for V are smaller than ∼0.01, smaller than ∼0.02 for R and smaller than ∼0.04 for I. Riello and collaborators only gave transformations between the Gaia magnitudes and the V, R and I magnitudes. In order to check the results of our transformations, we compare them with the measurements for a set of stars in the open cluster IC 1590. The transformed values have residuals smaller than ∼0.1 for all magnitudes except U where the residuals are smaller than ∼0.4. For stars fainter than +18 in all magnitudes the residuals are much larger because the faint limit of validity for these transformations is Mag ∼ 16, however, they seem to give reasonable results in a small range fainter than 16.