The effect of allowing the dark matter core size to freely vary when fitting data for WLM. The lines and symbols are as in Fig. 2. Recall that the dark matter core size is set by equation (21): r_c = ηR_1/2 and is therefore controlled by the dimensionless parameter η. In this fit to WLM's rotation curve, we allow 0 < η < 5; the histogram of recovered η from our model chains is shown in the bottom right panel. Notice that the data clearly favour a dark matter core over a cusp (η = 0), while our favoured value of η = 1.75 from R16 is recovered at the edge of our 68 per cent confidence intervals.