Context :

Aims : A long-timeline kinematic study of the archetypal compact symmetric object ( CSO ) OQ 208 sheds light on the physical properties of the most compact radio sources .

Methods : Archival data from the Very Long Baseline Array ( VLBA ) at 15 GHz over a time span of 13.6 yr were used to investigate the kinematics of the radio source .
The flux density monitoring data obtained at the Michigan 26-meter radio telescope were also used as supplementary information for analyzing the geometry of the radio structure .

Results : At 2.3-GHz , the radio emission is dominated by two mini-lobes separated by \sim 10 pc in a northeast-southwest ( NE–SW ) direction .
At 8.4 and 15 GHz , each lobe is further resolved into two subcomponents , which are identified as hotspots .
A knotty jet is linked with the NE hotspot and traces back toward the geometric center .
The core is too weak to be detected .
Significant flux density variation is found in the primary hotspots with a maximum level of 62 % ( NE1 ) and 19 % ( SW1 ) .
The flare epoch of NE1 is earlier than that of SW1 by approximately 5.00 yr , suggesting that the northeast lobe is advancing and the southwest lobe is receding .
This light travel difference indicates a radial distance difference between the two hotspots of 1.53 pc , which indicates an inclination angle of about 80.8 degrees between the radio jet and the line of sight .
The angular separation rate between NE1 and SW1 is 0.027 mas yr ^ { -1 } , corresponding to a projected speed of 0.133 c. The inner jet knot ( J1 ) moves at 0.047 mas yr ^ { -1 } ( or 0.230 c ) , about 3.5 times the hotspot advancing speed .

Conclusions : The large viewing angle and the modest jet speed suggest a mildly relativistic jet .
The jet axis is close to the plane of the sky .
The separation rate and the distance between the two primary hotspots result in a kinematic age of 255 \pm 17 yr , confirming that OQ 208 is indeed a young radio source .
In addition to the hotspot advancing motions , sideways motions provide evidence that the lobes are obstructed by the external interstellar medium .