Transformations between the <i>Gaia G</i>BP, <i>G</i>, and <i>G</i>RP and the Johnson–Cousins photometric system

ABSTRACT We used the Gaia $G_{\rm BP}$, G, and $G_{\rm RP}$ 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 $8 &amp;lt; {\rm Mag} &amp;lt; 16$. The values of the residuals are smaller than $\sim 1.0$ for U, smaller than $\sim 0.1$ for B, and smaller than $\sim 0.05$ 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 $\sim 0.01$, smaller than $\sim 0.02$ for R, and smaller than $\sim 0.04$ 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 ${\rm IC }\ 1590$. The transformed values have residuals smaller than $\sim 0.1$ for all magnitudes except U where the residuals are smaller than $\sim 0.4$. For stars fainter than $+18$ in all magnitudes the residuals are much larger because the faint limit of validity for these transformations is ${\rm Mag} \sim 16$, however, they seem to give reasonable results in a small range fainter than 16.