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We present first epoch, polarization sensitive 5~GHz VLBI observations of four X-ray selected BL~Lacertae objects (XBL) from the HEAO-1 catalog. All targets show a core-jet morphology similiar to that observed in radio-selected BL~Lacertae objects (RBLs). Polarized jet features suggest that the parsec-scale magnetic field orientation in XBLs differs from that usually seen in RBLs. It is commonly believed that BL~Lacertae objects are intrinsically low luminosity radio galaxies with relativisitic jets oriented close to the line of sight, with XBLs being at larger angles to the line of sight than RBLs. However, the ratio of the parsec-scale core and kiloparsec-scale extended radio fluxes in XBLs and RBLs differ from the expectations of this unified scheme and simple beaming models. The difficulty may lie in the assumption that XBL and RBL jets are intrinsically identical. We find that the jets in XBLs fade more quickly than in RBLs, suggesting the importance of factors other than orientation.
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