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We address the topic of the Intra-Night Optical Variability of the BL Lacobject S5 0716+714. To this purpose a long term observational campaign wasperformed, from 1996 to 2003, which allowed the collection of a very large dataset, containing 10,675 photometric measurements obtained in 102 nights. Thesource brightness varied in a range of about 2 mag, although the majority ofobservations 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 exponentiallaw with a mean value of 0.027 mag/h, corresponding to an e-folding time scaleof the flux tau_F = 37.6 h. The highest rates of magnitude variation werearound 0.10--0.12 mag/h and lasted less than 2 h. These rates were observedonly when the source had an R magnitude < 13.4, but this finding cannot beconsidered significant because of the low statistical occurrence. Thedistribution of tau_F has a well defined modal value at 19 h. Assuming therecent estimate of the beaming factor delta about 20, we derived a typical sizeof the emitting region of about 5 times 10^{16}/(1 + z) cm. The possibility tosearch for a possible correlation between the mean magnitude variation rate andthe long term changes of the velocity of superluminal components in the jet isdiscussed.Comment: 12 pages, 10 figures, accepted for Astronomy and Astrophysic

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