Extreme synchrotron BL Lac objects - Stretching the blazar sequence

We performed an observational program with the X-ray satellite BeppoSAX, to study objects with extreme synchrotron peak frequencies (nu (peak) > 1 keV ). Of the seven sources observed, five revealed or confirmed their extreme nature. Four showed peak frequencies in the range 1-5 keV, while one (1ES 1 426+428) displayed a at power law spectrum (alphax = 0.92 +/-0.04) which lo cates its synchrotron peak at or above 100 keV. This is the third source of this type ever found, after Mkn 501 and 1ES 2344+514. In the context of th e whole blazar class, the broad band properties of these objects confirm th e scenario of a synchrotron peak smoothly spanning the IR - X-ray range, wh ich explains the multi-frequency properties of the blazar class. Our data a lso confirm the large nu (peak) variability which seems to characterize thi s class of sources, compared with lower nu (peak) objects. Given the high s ynchrotron peak energies, which ag the presence of high energy electrons, t hese extreme BL Lacs are also good candidates for TeV emission, and therefo re good probes of the IR background.