1996A&AS..120C.495W

A fraction of the blazars discovered by EGRET has shown rapid variations in the gamma-ray range during an individual CGRO viewing period. Many of the EGRET blazars are known to be rapidly variable at lower photon energies as well. We have observed ten of the gamma-bright blazars in the optical regime simultaneously during EGRET measurements. A few of these campaigns include polarimetric and spectral-index observations. Some of them were monitored during several EGRET observations. All of the objects have varied in the optical range. Variations are fast and need at least daily sampling. The changes in flux density are generally accompanied by changes of spectral indices and polarization properties. This demonstrates that the conditions leading to the synchrotron emission has changed during an individual gamma-ray observations. We discuss evidence for comparable changes of the gamma-ray radiation, which is often plagued by the low photon statistics. We also discuss the implications of the variability for interpretations of the broad-band energy distributions and the relevance of these observations for models of gamma-ray emission. The evidence for a close link of gamma-ray emission and synchrotron radiation is discussed.