SEDs of selected C3PO sources with temperatures T_c < 8K. Black diamonds with error bars are the IRIS 3THz and Planck 857, 545 and 353GHz data. Two fits have been performed: one with β = 2 (blue); and the other with β free (red). For comparison with the data (black diamonds), the expected flux densities for the two fits, with the same colour convention, are shown with blue squares and red plus signs. The quality of each fit can be judged by comparing the estimated flux densities in each band, i.e., blue squares and red plus signs, with the actual measurements, i.e., black diamonds with error bars. Two models are overlaid to define an approximate error envelope in the case of β free: the (T + σ_T, β − σ_β) modified blackbody emission model, in the red dotted curve; and the (T − σ_T, β + σ_β) model in the red dashed curve.