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

We report intensity variations of different spectral components in the BL L ac object S5 0716+714 detected during coordinated BeppoSAX and optical obse rvations in 1996 and 1998. The transition between synchrotron and inverse C ompton 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 variab ility was found during the second BeppoSAX pointing when intensive optical monitoring could be arranged. The hard (Compton) component changed by a fac tor of 2 between the two observations, but remained stable within each expo sure. During events of rapid variability S5 0716+714 showed spectral steepe ning with intensity, a behaviour rarely observed in BL Lacs. We interpret t hese findings as the probable consequence of a shift of the synchrotron pea k emission from the IR/optical band to higher energies, causing the synchro tron tail to push into the soft X-ray band more and more as the source brig htens.