Extended Data Fig. 1: Pristine photometric information for all stars of the C-19 stream selected by STREAMFINDER and present in the Pristine survey.

Large symbols represent stars with G₀ < 19.0, for which the STREAMFINDER selection is very reliable, and small symbols represent stars fainter than this limit, which are more likely contaminated in the STREAMFINDER catalogue (see also Fig. 1). The lines represent model expectations as determined from the spectral libraries and filter response curves, without an assumption on whether the star is a dwarf or a giant¹⁰. Both model lines and data points are color-coded by their \({[{\rm{Fe}}/{\rm{H}}]}_{{\rm{Pristine}}}\) metallicities. Most C-19 candidate members are located in the region that corresponds to metallicities below \([{\rm{Fe}}/{\rm{H}}]=-3.0\) (above the blue line). For the large data points, we specifically used a photometric metallicity model tailored to giant stars. Near the tip of the red giant branch, it deviates significantly from the generic model represented by the coloured lines and explains the higher metallicity of the right-most point compared with the models.