This research has made use of the APS catalogue of POSS I database which is supported by the National Science Foundation, the National Aeronautics and Space Administration, and the University of Minnesota
We are grateful to E. Flesch for extracting for us the recalibrated O magnitudes from the APM and USNO catalogues and to E. Flesch, F. Ochsenbein and R. Monella for having brought to our attention a number of errors and omissions in the 8th edition of this catalogue.