SIMBAD references

The Photometric Magnitude Difference Catalog, maintained at the US Naval Observatory, is a collection of magnitude-difference measures for double stars and serves as a repository for double star observations where no astrometry is given. At the present time, it consists of 19,594 measures of 10,475 systems with a mean Δm of 1.90. Because of the nonuniformity of filters and the possible variability of one or more components in a system, no ``WDS-like'' summary line is provided. Rather, all the data are presented and their interpretation is left as an ``exercise for the reader.'' This catalog is one of four USNO double star catalogs to be included on a new CD-ROM. A brief summary and statistical analysis of the contents of the catalog are presented.