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tokens: ± 0.10, assuming no change in flux between the two observations), similar to the compact jets of other X-ray binaries, although we cannot rule out a steep spectrum within uncertainties. We find that the spectral index measurements within the 10 GHz band on the other observing epochs are typically unconstraining (with a typical error of ±0.5, due to the faintness of the radio emission and the small lever arm in frequency), so we cannot distinguish between optically thin and thick emission. The spectral index measurements for all epochs are listed in Table 3 . 4.5 The radio–X-ray correlation The radio ( L R = 4π D 2 ν S ν , where D is the distance to the source) and X-ray luminosities of our sample are displayed in Fig. 5 individually, and together in Fig. 6 , with the addition of other black hole and neutron star systems from the literature. Figure 5. The radio and X-ray luminosities of Cen X−4, SAX J1808.4−3658 and IGR J00291+5934. Hollow symbols mark upper limits. The labelled data points of SAX J1808.4−3658 refer to the same observations in Fig. 4 . IGR J00291+5934 (and possibly SAX J1808.4−3658, but with larger scatter) seems to follow the familiar correlation seen in black holes and some atoll sources, with β = 0.77 ± 0.18 (90 per cent confidence interval, shaded area). Positions of X-ray binaries on the radio–X-ray plane are best measured from strictly simultaneous observations. Among our multiwavelength campaign, Cen X−4 has strictly simultaneous data. For the other sources, we interpolate the X-ray light curve to estimate the X-ray flux at the time of the radio observations, by fitting a piecewise linear function in the log L X /time space. The 3 σ upper limit of 14 μJy on the radio emission of Cen X−4 in quiescence only constrains its radio luminosity to being less than that of the radio-fainter black hole jets (e.g. XTE J1118+480; Gallo et al. 2014 ). The limit lies just above the proposed tMSP track, slightly below the quiescent black hole systems. In addition, we also place the 1979 outburst of Cen X−4 on the radio/X-ray plane for the first time. For the radio luminosity, we use the data reported by Hjellming et al. ( 1988 ) in two bands (at 1.49 and 4.9 GHz). For days when observations were carried out in both radio bands, we use the measurement with smaller uncertainties, with the assumption of a flat spectrum. We find that during the 1979 outburst, Cen X−4 reached radio and X-ray luminosities similar to the atoll sources. Assuming it follows a single, continuous radio–X-ray correlation from the peak of the outburst down to quiescence (like the black hole systems V404 Cyg, Corbel et al. 2008 ; and GX 339–4, Corbel et al. 2013 ), we obtain a lower limit for its correlation slope β ≳ 0.5. The upper limits for IGR J17511–3057 make it less luminous in the radio band than typical atoll sources (Fig. 6 ), but we cannot rule out radio emission at a level similar to that of EXO 1745−248, the radio-detected neutron star with the lowest