Carte du Ciel and Gaia I. Astrometry

Context. The Carte du Ciel archive at the University of Helsinki enables us to see the sky as it was about 120 yr ago. The archive consists of single-exposure and triple-exposure plates between epochs 1896.8-1925.8.Aims. Our main aim is to find binary and multiple stars by combining Carte du Ciel and Gaia data. Methods. The plates were digitised with a commercial digital camera. We used Gaia data to calculate predicted coordinates of stars at the epoch of each plate. These stars were used as reference stars to fit astrometry for each plate, giving fitted coordinates for stars on the Carte du Ciel plates. If the predicted and fitted coordinates differed at a significant level, we classified the star as a non-single star, for which the proper motion values given in the Gaia catalogue can be unreliable. Results. 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 catalogues of known binary stars, is relatively low, similar to 10% at maximum. Conclusions. 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.