SIMBAD references

Photographic plates of the globular cluster M10 (NGC 6254) that were recorded at the Dominion Astrophysical Observatory between 1931 and 1934 have been digitized with a commercial scanner. The plates have numerous cosmetic issues, and the results are used to assess the information that can be extracted from such archival data. After performing a series of basic processing steps, a plate-to-plate dispersion in photometric measurements of ≤10% is delivered between V = 13 and 15. The light curves of known variables are constructed and evidence is presented for long-term variations in the mean brightnesses of the W Vir star V3, as well as the semiregular (SR) variable V29. Two new variable stars are identified, although cluster membership is likely for only one of these. A brightness-selected sample of candidate variable stars identified in the Gaia database that are not in the General Catalogue of Variable Stars is also examined, and some show variability in the plate photometry. Parallaxes indicate that five of the Gaia variables are at the same distance as M10. Four of the Gaia variables fall along the horizontal branch of M10 on the color-magnitude diagram. Another Gaia variable is located close to the cluster center and may be evolving on the supra-horizontal branch. The plate photometry suggests that the Gaia variable with the most secure M10 membership based on parallax, proper motions, and velocity is an SR variable that is offset by a projected distance of 9 pc from the cluster center.