Large photometric or spectroscopic surveys are used to sort stars into populations and define the main trends that characterise them, as diagnostics of their origin. Stars falling off the trends defined by the ‘normal’ stars are called ‘peculiar’ and typically eliminated in discussions of Galactic structure and evolution. In our programme on extremely metal-poor halo giants, we have recently focused on the small subgroup that is strongly enhanced in r-process elements, asking whether the chemical peculiarity is intrinsic to these stars or due to local surface pollution caused by mass transfer from a binary companion. Precise radial-velocity monitoring over several years turns out to disprove the binary hypothesis and has led to new insight in the processes of chemical enrichment in the early Galactic halo. An ongoing analogous programme on carbon-enhanced metal-poor giants is briefly described at the end.
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