Radio spectra of the total emission of the remnant candidates. Flux densities from LOFAR18 at 150, NVSS at 1400 and VLA at 6000 MHz are shown with darker blue symbols, while GMRT at 325 MHz and VLA in-band spectral measurements centred at 4500, 5500, 6500 and 7500 MHz are shown in lighter blue symbols. For the source J1057+5911, we use the WSRT upper limit at 1400 MHz. Upper limits are indicated with triangles. The black line connects the measurements used for the calculation of the spectral index for sources detected at 150, 1400 and 6000 MHz. Flux density scale errors are smaller than the size of the symbol representing the flux density measurement.