The intra-night optical variability of the bright BL Lacertae object S5 0716+714

Aims. We address the topic of the intra-night optical variability of the BL Lac object S5 0716+714. Methods. To this purpose a long-term observational campaign was carried out, from 1996 to 2003, which allowed the collection of a very large data set, containing 10675 photometric measurements obtained in 102 nights. Results. The source brightness varied in a range of about 2 mag, although the majority of the observations were performed when it was in the range 13.0 < R < 13.75. Variability time scales were estimated from the rates of magnitude variation, which were found to have a distribution function well fitted by an exponential law with a mean value of 0.027 mag/h, corresponding to an e-folding time scale of the flux $\tau_F = 37.6$ h. The highest rates of magnitude variation were around 0.10-0.12 mag/h and lasted less than 2 h. These rates were observed only when the source had an R magnitude