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 <13.4, but this finding cannot be considered significant because of the low statistical occurrence. The distribution of $\tau _F$ has a well-defined modal value at 19 h. Assuming the recent estimate of the beaming factor $\delta \sim 20$, we derived a typical size of the emitting region of about $5\times 10^{16}/(1 + z)$ cm. The possibility of searching for a possible correlation between the mean magnitude variation rate and the long-term changes in the velocity of the superluminal components in the jet is discussed.