Carte du Ciel and Gaia. I. Astrometry.

If the predicted and fitted coordinates differ at a significant level, we classify the star as a non-single star, for which the proper motion values given in the Gaia catalogue can be unreliable. We find that several astrometric quality indicators of Gaia indicate that the uncertainties of Gaia's single-star model fit are in general larger for our non-single star candidates. The percentage of our non-single star candidates which are in the catalogs of known binary stars is relatively low, about 10% at maximum.

The combination of the Carte du Ciel and Gaia data can be used to identify candidates of non-single stars. We propose that the sources with a significant difference between the predicted and fitted coordinates are long-period binaries, although astrophysical and/or instrumental effects as origin for the coordinate difference cannot be excluded for individual cases.