Superluminal motion in a compact steep spectrum radio source 3C 138

We present the results of 5 GHz VLBI observations of a compact steep spectr um source 3C 138. The data are consistent with the western end being the lo cation of the central activity. The observed offset between different frequ encies in the central region of 3C 138 can be accounted for by a frequency dependent shift of the synchrotron self-absorbed core. Our new measurements confirm the existence of a superluminal motion, but its apparent velocity of 3.3c is three times slower than the reported one. This value is consiste nt with the absence of parsec-scale counter-jet emission in the inner regio n, but seems still too high to allow the overall counter-jet to be seen in terms of Doppler boosting of an intrinsically identical jet. Either an inte raction of jet with central dense medium, or an intrinsically asymmetrical jet must be invoked to reconcile the detected superluminal speed with the o bserved large scale asymmetry in 3C 138.