2000A&A...363..108V

In February 1999, the Whole Earth Blazar Telescope (WEBT) organization carried out an optical campaign on the blazar S5 0716+71. This campaign had two principal aims: to test the potential of such an optical telescope network devoted to the study of blazar variability and to search for possible time delays between variations in different optical bands. The main results of this campaign are presented here. The six participating observatories collected a large amount of optical data in UBVRI bands: 635 observations were performed in the observing period from February 16.0 to 19.0, forming the densest dataset ever obtained for a blazar during a 72-hour period. Besides constructing continuous and dense optical light curves, this monitoring effort has provided some very significant results. On one hand, we have determined the steepest recurrent variation slope to be 0.002mag per minute, on both rising and decreasing phases, thus constraining the properties of the emitting region. On the other hand, an upper limit to the possible delay between B and I variations (10 minutes) has been fixed thanks to the high quality and density of part of the dataset. Moreover, in a comparison with previous data, we have discovered a long-term trend of the spectral index, implying a steepening of the optical spectra and a shift of the synchrotron peak towards the infrared during the last five years.