Left: overlay of the 1.5 (black contours), 5 (white contours), and 8.4 GHz (blue contours) emission of PSO J0309+27. The greyscale map is the 1.5 GHz self-calibrated image, and its restoring beam is shown in the bottom left corner; north is up, and east is left. The phased referenced absolute position of component 1a is 03^h09^m47.4862^s, +27°17′57.3165″. Right: radio spectrum of all the sub-components of PSO J0309+27 spatially resolved by the multi-frequency VLBI observations presented here. We indicate the 3σ detection limit for sub-component 4 at 5 and 8.4 GHz with arrows, estimated as three times the flux density within the same area of the 1.5 GHz detection of that sub-component.