Radio (8.5 GHz) luminosity as a function of X-ray (2–10 keV) luminosity of NS XRBs (adapted from Migliari et al. 2011). Colours: yellow, accreting millisecond pulsars; red, atoll sources in the hard state or in outburst; blue, atoll sources steadily in the soft state; green, Z sources. Markers: 4U 1728−34, open circles; Aql X–1, open squares; MXB 1730–335, filled squares; 4U 0614+091, filled circles; 4U 1820−30, filled stars; Ser X–1, open star. The new observations presented in this paper are marked with black circles. Note that for 4U 1820−30 we show the extrapolation of the 5 GHz flux to 8.5 GHz assuming an inverted spectrum with a slope of index 0.13, as derived in Sect. 3.2 instead of the upper limit from the observations reported here.