Illustration of the β − T degeneracy: the panels on the left-hand side show fits to the 4 data points to which noise has been added, and the right column shows the values of β and T. The large triangles on the left show the original data points (before adding noise), which follow a modified black-body law with β = 1.8 and T = 18, typical values for normal spiral galaxies. The large open stars show this position in the right-hand column. From bottom to top, the noise added follows a Gaussian distribution with standard deviation of 5% (bottom panels), 2% (middle panels) and 1% (top panels). Despite this very low noise level, the scatter in β and T is significant and correlated. An enlargement of the spectral coverage would help reduce the degeneracy if no other processes such as emission from small or spinning dust grains play a role within the extended wavelength range.