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Context. Monitoring of BL Lacertae at 15 GHz with the Very Long BaselineArray (VLBA) has revealed a quasi-stationary radio feature in the innermostpart of the jet, at 0.26 mas from the radio core. Stationary features are foundin many blazars, but they have rarely been explored in detail. Aims. We aim tostudy the kinematics, dynamics, and brightness of the quasi-stationary featureof the jet in BL Lacertae based on VLBA monitoring with submilliarcsecondresolution (subparsec-scales) over 17 years. Methods. We analysed positionuncertainties and flux leakage effects of the innermost quasi-stationaryfeature and developed statistical tools to distinguish the motions of thestationary feature and the radio core. We constructed a toy model to simulatethe observed emission of the quasi-stationary component. Results. We find thattrajectories of the quasi-stationary component are aligned along the jet axis,which can be interpreted as evidence of the displacements of the radio core.The intrinsic motions of the core and quasi-stationary component have acommensurate contribution to the apparent motion of the stationary component.During the jet-stable state, the core shift significantly influences theapparent displacements of the stationary component, which shows orbiting motionwith reversals. The quasi-stationary component has low superluminal speeds ontime scales of months. On time-scales of few years, the apparent mean speedsare subrelativistic, of about 0.15 the speed of light. We find that thebrightness profile of the quasi-stationary component is asymmetric along andtransverse to the jet axis, and this effect remains unchanged regardless ofepoch. Conclusions. Accurate positional determination, a high cadence ofobservations, and a proper accounting for the core shift are crucial for themeasurement of the trajectories and speeds of the quasi-stationary component.(abbrev.)

Studies of stationary features in jets: BL Lacertae. I. Dynamics and brightness asymmetry on sub-parsec scales