XMM-Newton observation of a sample of HBL BL Lacs with extreme SEDs

We propose to observe with XMMNewton 13 BL Lacs to perform an Xray andmultifrequency study of a complete sample of extreme highenergypeaked HBLBL Lacs. These sources have been chosen from the Sedentary Survey, the largestexisting fluxlimited and complete sample of BL Lacs, to be those with thehighest fx/fr ratio for which a detailed Xray spectrum is not yet available. They are part of a larger and well defined sample partly observed with BeppoSAX.We will combine their Xray spectra with OM optical grism data and observationsat other frequencies to determine their SEDs. This will allow us to make astatistically meaningful study of the particle acceleration and emissionmechanisms in a welldefined sample of BL Lacs. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]