HR diagrams illustrating the error box corresponding to the observations derived in this work for HD 140283. The left and right panels show the results when we adopt A_V = 0.00 and 0.10 mag. The dotted box in the left panel shows the error box when the distance is increased by 1σ, also illustrated by the arrow. In the right panel (non-zero reddening solution) we also show the A_V = 0.00 error box (dotted box), located in the lower right part of the figure. The central models correspond to Y_i = 0.245,α = 1.00, and Z/X_i = 0.0005 (with Re_ν = 5). The optimal masses are M = 0.780 and M = 0.805M_⊙, and are shown as the blue continuous lines passing near the centre of the error boxes. We also illustrate other models by changing the mass (blue) and the other parameters (green). The red dotted line corresponds to the radius derived in this paper. See Sect. 7.3 for details.