Résumé
Aims.We analyse two samples of high latitude carbon stars found in two objective-prism surveys: the First Byurakan survey (FBS) and the Hamburg/ESO survey (HES). We determine the composition of these samples in cool AGB stars, warmer giants and dwarf carbon stars. Methods: We list 44 new carbon stars found in the FBS and use near-infrared photometric catalogues (DENIS, 2MASS) to investigate the observational and statistical properties of the surveys. The SuperCOSMOS and USNO-B1.0 catalogues are used to study proper motions and to identify dwarfs. Results: The colour-magnitude and colour-colour diagrams, either with visual magnitudes or with infrared data, show that the HES selects faint and relatively warm carbon stars, whereas FBS is less deep, but gives cooler objects. By comparing DENIS and 2MASS photometry, we find that HES objects with J-K ? 1 are mostly non-variable giants while clear variability is often seen for J-K ? 1, indicating AGB stars. By using SuperCOSMOS proper motions, 9 HES objects are found to have a proper motion ? greater than 30 mas yr-1 (3? detection). Five of these objects are detected at the ?5? level. These 9 HES objects are dwarf carbon stars, including two that could belong to the halo population. The 9 carbon dwarfs are remarkably bright and deserve further study. Partly based on observations done at ESO, Chile (program 69.B-0086), at Byurakan Observatory (Armenia), and at Haute Provence Observatory (France) operated by the Centre National de Recherche Scientifique. Appendices and Table 1 are only available in electronic form at http://www.aanda.org