Left: overall uncertainty (statistical and systematic) that is due to the considered variations in α_ml, in dependence on the mass of the primary star. The lighter region shows the uncertainty on the median of the estimated β that is a result of ignoring the correct mixing-length value. The intermediate colour regions show the overall error up to 1σ caused by the cumulated contribution of observational errors and systematic bias that is due to the unknown mixing-length value. The darker regions correspond to errors up to 2σ. Right: same as in the left panel, but in dependence on the relative age of the primary star.