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Published online by Cambridge University Press: 03 August 2017
During the 1970's, the compact radio structure of 4C39.25 consisted of a simple double source, whose separation did not change. A new component, which probably originated in mid-1979, is now seen to be moving superluminally between the original, still stationary, components. It is not clear which component of the source is the core, nor how the moving component will interact with the stationary component that it is now approaching on a collision course.