Abstract. The Hamburg RASS X-ray bright BL Lac sample (HRX-BL Lac) consists of 106 X-ray selected objects, with 77 BL Lacs forming a complete sample. The candidates have been selected from a cross-correlation of the ROSAT All-Sky Survey with the NVSS radio catalogue. This selection results in an X-ray dominated sample of high frequency cut-off BL Lac objects. The large number of objects allows detailed studies of the dependencies of physical parameters on the spectral energy distribution. The complete sample is used to study the evolution of the different subtypes of BL Lac objects.

A scenario, in which BL Lacs start as luminous low frequency cut-off objects and then fade to the state of high frequency cut-off, is proposed on the basis of the different evolutionary behaviour and on the different observed quantities.

Within the course of identification also a possible ultra-high frequency cut-off BL Lac has been found, which could be the counterpart of an up to now unidentified EGRET source. This object could be a promising target for an INTEGRAL observation.

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