New extreme synchrotron BL Lac objects

We report on the BeppoSAX observations of four ``extreme'' BL Lacs, selected to have high synchrotron peak frequencies. All have been detected also in the PDS band. For 1ES 0120+340, PKS 0548-322 and H 2356-309 the spectrum is well fitted by a convex broken power-law, thus locating the synchrotron peak around 1-4 keV. 1ES 1426+428 presents a flat energy spectral index (alpha<SUB>x</SUB>=0.92) up to ~100 keV, thus constraining the synchrotron peak to lie near or above that value. For their extreme properties, all sources could be strong TeV emitters. .