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PKS 1749+096 is a BL Lac object showing weak extended jet emission to thenortheast of the compact VLBI core on parsec scales. We aim at betterunderstanding the jet kinematics and variability of this source and findingclues that may applicable to other BL Lac objects. The jet was studied withmulti-epoch multi-frequency high-resolution VLBI observations. The jet ischaracterized by a one-sided curved morphology at all epochs and allfrequencies. The VLBI core, located at the southern end of the jet, wasidentified based on its spectral properties. The equipartition magnetic fieldof the core was investigated, through which we derived a Doppler factor of 5,largely consistent with that derived from kinematics (component C5). The studyof the detailed jet kinematics at 22 and 15 GHz, spanning a period of more than10 years, indicates the possible existence of a bimodal distribution of the jetapparent speed. Ballistic and non-ballistic components are found to coexist inthe jet. Superluminal motions in the range of 5-21 c were measured in 11distinct components. We estimated the physical jet parameters with the minimumLorentz factor of 10.2 and Doppler factors in the range of 10.2-20.4 (componentC5). The coincidence in time of the component's ejection and flares supportsthe idea that, at least in PKS 1749+096, ejection of new jet components isconnected with major outbursts in flux density. For the best-traced component(C5) we found that the flux density decays rapidly as it travels downstream thejet, accompanied by a steepening of its spectra, which argues in favor of acontribution of inverse Compton cooling. These properties make PKS 1749+096 asuitable target for an intensive monitoring to decipher the variabilityphenomenon of BL Lac objects.

The parsec-scale jet of PKS 1749+096