The 0716+714 WEBT campaign of February 1999

In February 1999, the Whole Earth Blazar Telescope (WEBT) organization carr ied 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 colle cted a large amount of optical data in UBVRI bands: 635 observations were p erformed in the observing period from February 16.0 to 19.0, forming the de nsest dataset ever obtained for a blazar during a 72-hour period. Besides c onstructing continuous and dense optical light curves, this monitoring effo rt has provided some very significant results. On one hand, we have determi ned the steepest recurrent variation slope to be 0.002 mag per minute, on b oth rising and decreasing phases, thus constraining the properties of the e mitting region. On the other hand, an upper limit to the possible delay bet ween B and I variations (10 minutes) has been fixed thanks to the high qual ity and density of part of the dataset. Moreover, in a comparison with prev ious data, we have discovered a long-term trend of the spectral index, impl ying a steepening of the optical spectra and a shift of the synchrotron pea k towards the infrared during the last five years.