The extreme high frequency peaked BL Lac 1517+656

We present optical spectroscopy data that allowed a measurement of the redshift for the X-ray selected BL Lacertae object 1517+656. With a redshift of z = 0.702 this object has an absolute magnitude of M_B = -26.4 and is also an extremely powerful radio and X-ray source. Although being a high frequency peaked BL Lac, this object is one of the most luminous BL Lac objects known so far. Being also a candidate for gravitational lensing, this object is of high interest for the BL Lac research. Assuming several cosmological models and a realistic redshift for the lensed object, we find that 1517+656 has a mass > 2 * 10¹² M_sun and a high velocity dispersion > 350 ; km ; sec^-1. Based on observations from the German-Spanish Astronomical Center, Calar Alto, operated by the Max-Planck-Institut für Astronomie, Heidelberg, jointly with the Spanish National Commission for Astronomy