VERY RAPID OPTICAL VARIABILITY OF PKS-2155-304

We have performed an optical observation campaign on PKS 2155-304, who se aim was to determine the variability properties of this object on v ery short time scales in several photometric bands. We detected variab ility on time scales as short as 15 min. The Fourier properties of the light curves have been investigated using structure function analysis . The power spectra are well described by a power-law with an index -2 .4. It is compatible with the index found in the X-ray domain. The val ue of this index shows that the light curves cannot be generated by a sum of exponential pulses. Using historical data, we find that the lon gest time scale of variability in the optical domain lies between 10 a nd 40 days. We find a strong correlation between flux and spectral ind ex, which we interpret as the signature of an underlying constant comp onent. As a result we do not find evidence of spectral variation for t he active nucleus in the optical domain. A lag has been found between the light curves in different optical bands. The short-wavelength ligh t curves lead the long-wavelength ones. The amplitude of the lag is ab out 40 min for a factor 2 in wavelength. Our results are compared with predictions from different models. None of them can explain naturally the set of results obtained with this campaign, but we bring out some clues for the origin of the variability.