Probability distribution of the surface helium mass fraction (Y_s, left panel) and surface nitrogen mass fraction (N_s) enhancement (right panel, see text) of donor vs. the accretor that is predicted to be observed in the semi-detached configuration of the Case A mass transfer phase based on the model grid. The different coloured ‘stars’ with error bars denote the position of the semi-detached systems of the TMBM survey and Galaxy. The surface nitrogen mass fraction enhancement of the LMC systems are evaluated w.r.t. to the LMC nitrogen abundance baseline, while the enhancement for LZ Cep and XZ Cep are evaluated w.r.t. to the Solar baseline. The grey-scale gives the probability fraction in each pixel. The total probability is normalised such that the integrated sum over the entire area is 1. The orange line indicates where the surface helium mass fraction or surface nitrogen enrichment of the donor and accretor is the same.