HRD for BD+20 5391. Component A is marked in red and component B in blue. The squares denote the parameters derived from the SED and spectral fitting, while triangles indicate the onset of RLOF. The best-fit MIST evolutionary tracks are represented by discrete dots extending from the zero-age main sequence to the tip of the RGB. To visualise the uncertainties, we also show a random subset of tracks from our MCMC calculation.