Résumé
Aims.The goal of this paper is to present and analyse a new sample of cool carbon (C) stars located in the Galactic halo. Methods: These rare objects are discovered by searching the 2MASS point-source catalogue for candidates having near-infrared colours typical of C stars. Optical spectroscopy is subsequently performed. Results: Twenty-three new C stars were discovered, with their Ks magnitude in the range 6 to 13.3. Spectra are typical of N-type carbon stars with C{2} and CN bands and sometimes H? in emission. One object is a S-type star. When the objects are bright enough (V ? 15.5), the data of the Northern Sky Variability Survey can be exploited. In all cases, stars belonging to this survey show light variations confirming that they are AGB stars. Distances and galactocentric XYZ coordinates have been estimated by assuming these stars to be similar in luminosity to those of the Sagittarius dwarf galaxy. Four objects are particularly red with J-Ks > 3, with two located at more than 5 kpc from the Galactic plane. Eight additional objects with similar properties are found in the literature and our previous works. These 12 C stars could be useful to study mass loss at low metallicity. Two other objects are remarkably far from the Sun, at distances of 95 and 110 kpc. They are located, together with two other C stars previously found, in the region Z < -60 kpc in which the model of Law et al. (2005) predicts the Sgr Stream to have a loop. Based on observations done at Haute Provence Observatory operated by the Centre National de Recherche Scientifique (France), and at Byurakan Observatory (Armenia). Appendix A is only available in electronic form at http://www.aanda.org