SEDs of the four sources in our sample with extremely steep spectral indices. The curve from Eq. (3) was fitted to the radio flux density measurements available for these sources and is indicated as a black curve. Upper left: ILTJ144448.65+560615.6, which has a spectral index between LoLSS and LoTSS of α_low = 2.5 ± 0.4. Upper right: ILTJ151214.28+542710.1 with a spectral index between LoLSS and LoTSS of α_low = 2.9 ± 0.5. Lower left: ILTJ132201.76+564652.5 with a spectral index between LoLSS and LoTSS of α_low = 2.5 ± 0.3. Lower Right: 87GB B0903+6656 (ILTJ090723.44+664446.6), with a spectral index between LoTSS and NVSS of α_low = 2.7 ± 0.1. For this source, we additionally plot a 4.58 GHz observation taken by the NRAO Green Bank telescope (Becker et al. 1991), and an 8.4 GHz observation taken by the VLA (Healey et al. 2007).