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Published online by Cambridge University Press: 19 July 2016
The coordinate system of the Catalogue of Positions and Proper Motions (PPM) was constructed as an extension of the FK5 sytem to higher star densities and fainter magnitudes. Zonal deviations from the FK5 were minimised on scales of 7 × 7 square degrees via AGK3R and CMC catalogues. Compared with these catalogues, no magnitude equation was found for stars brighter than m(pg) = 10.5.