Colour-absolute magnitude diagram of UBC 1 with a selection of reddened rotating MIST isochrones (v/v_crit = 0.5) in blue. The best-fitting isochrone is given in orange and corresponds to the values indicated in Fig. 3. The dotted horizontal line shows the limit used in the fit. The uncertainties are typically within the symbol size. Crosses indicate potential binaries that were omitted from the fit.