long term variability of S5 0716+714 from the Asiago archive plates

We present the historic light curve of the BL Lac object S5 0716+714, spanning the time interval from 1953 to 2003, built using Asiago archive plates and our recent CCD observations. The source shows an evident long term variability, over which short term variations are superposed. We can model the source behaviour with an average monotonic brightness increase, since about 1975. This behaviour might be due to a precessing motion of the jet, as already suggested in the literature for other BL Lac objects. We remark the importance of preserving and digitizing the astronomical plate archives which are fundamental facilities to perform long term variability studies.