Radio spectra of the cores of the 21 remnant candidates. Flux density measurements from FIRST, VLASS and VLA images centred at 1400, 3000 and 6000 MHz, respectively, are shown with dark blue symbols, while triangles indicate upper limits. In light blue, we show VLA in-band measurements, centred at 4500, 5500, 6500 and 7500 MHz. Black lines connect the flux densities used for the calculation of the spectral index for the sources with detections at all frequencies. Flux density scale errors are smaller than the size of the symbol representing the flux density measurement.