Synchrotron and inverse Compton variability in the BL Lacertae object S5 0716+714

We report intensity variations of different spectral components in the BL Lac object S5 0716+714 detected during coordinated BeppoSAX and optical observations in 1996 and 1998. The transition between synchrotron and inverse Comp- ton emission has been clearly detected as sharp X-ray spectral breaks at around 2-3 keV on both occasions. Correlated optical and soft X-ray variability was found during the second Bep- poSAX pointing when intensive optical monitoring could be ar- ranged. The hard (Compton) component changed by a factor of 2 between the two observations, but remained stable within each exposure. During events of rapid variability S5 0716+714 showed spectral steepening with intensity, a behaviour rarely observed in BL Lacs. We interpret these findings as the prob- able consequence of a shift of the synchrotron peak emission from the IR/optical band to higher energies, causing the syn- chrotron tail to push into the soft X-ray band more and more as the source brightens.