Mosaic radio images at 400 MHz of the A399-A401 cluster pair. Left panel: high-resolution (12″ × 5″) mosaic image with σ_rms = 50 μJy beam⁻¹ produced with Briggs robust = 0 and primary-beam-corrected. A hint of diffuse emission from the radio halos is visible, but no emission is detected in the bridge area. Right panel: low-resolution (80″ × 80″), compact source-subtracted image with σ_rms = 600 μJy beam⁻¹. It is generated by Briggs robust=-0.5 and a Gaussian uv-taper of 60″. Contour levels start at 2σ_rms (in green) and increase up to 5σ_rms (black). The red-dashed box denotes the region where we find a 2σ_rms detection of a patch of the bridge. The red arrow points to the extension of the A399 radio halo, discussed in Sect. 3.3.