The long-term optical variability of the BL Lacertae object S5 0716+714: Evidence for a precessing jet

We present the historical 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, together with literature data. The source shows an evident long-term variability, over which well-known short-term variations are superposed. In particular, in the period from 1961 to 1983 the mean brightness of S5 0716+714 remained significantly fainter than that observed after 1994. Assuming a constant variation rate of the mean magnitude, we can estimate a value of about 0.11 mag yr(-1). The simultaneous occurrence of decreasing ejection velocities of superluminal moving components in the jet reported by Bach and coworkers suggests that both phenomena are related to the change of the direction of the jet to the line of sight from about 5 degrees to 0.degrees 7 for an approximately constant bulk Lorentz factor of about 12. A simple explanation is a precessing relativistic jet, which should at present be close to the smallest orientation angle. One can therefore in the next 10 years expect a decrease of the mean brightness of about 1 magnitude.