Colour-magnitude diagram track of DDI progenitor models for the 2015 LRN outburst in M31. The grey region with blue outline represents the observational uncertainties of the progenitor star, which include the distance modulus error, measurement error, and extinction uncertainty. The coloured lines show the CMD tracks of matching DDI models (which include the approximated companion photometry) during the last century before point F. The colours represent the time from any given point on the track to point F as indicated in the colour bar.