Cool carbon stars in the galactic halo

We briefly present an on-going survey to find AGB carbon stars in the halo. The main property of these objects is their high luminosity. In the galactic halo, about 100 such AGB C stars are known, and they are found up to a distance of similar to 150 kpc. The majority, but not all, belong to the tidal stream of the Sgr dwarf galaxy. They will contribute to characterize the stellar population originating from mergers, and they will also be useful to better determine the halo total mass and its dark matter content. How Gaia will see the various types of faint high latitude C stars (AGB, RGB or dwarf type) is briefly examined.