Long term follow-up coverage of Gaia photometric alert sources by Ondrejov robotic telescopes

The robotic telescopes at the Ondrejov Observatory are providing long-term optical multi-color coverage for selected 25 Gaia alert triggers located in the northern sky hemisphere. I will present and briefly discuss examples of selected results, mostly unpublished, obtained with these devices. In addition to that, I will present and discuss the potential of large historical photographic plate archives located around the globe as sources of both photometric as well as spectroscopic data. They allow us to perform the long-term study of photometric and spectroscopic evolution for astrophysical sources in general and for Gaia alert sources in particular. Some of these databases were digitized and on-line access is provided.