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We determined the fundamental properties of HD 140283 by obtaining newinterferometric and spectroscopic measurements and combining them withphotometry from the literature. The interferometric measurements were obtainedusing the visible interferometer VEGA on the CHARA array and we determined a 1Dlimb-darkened angular diameter of 0.353 +/- 0.013 milliarcseconds. Usingphotometry from the literature we derived the bolometric flux with twosolutions: a zero-reddening one of Fbol = 3.890 +/- 0.066 1E-8 erg/s/cm2 and asolution with a maximum of Av = 0.1 mag, Fbol= 4.220 +/- 0.067 1E-8 erg/s/cm2.The interferometric Teff is thus 5534 +/- 103 K or 5647 +/- 105 K and itsradius is R = 2.21 +/- 0.08 Rsol. Spectroscopic measurements of HD140283 wereobtained using HARPS, NARVAL, and UVES and a 1D LTE analysis of H-alpha linewings yields Teff(Halpha) = 5626 +/- 75 K. Using fine-tuned stellar modelsincluding diffusion of elements we then determined the mass M and age t ofHD140283. Once the metallicity has been fixed, the age of the star depends onM, initial helium abundance Yi and mixing-length parameter alpha, only two ofwhich are independent. We need to adjust alpha to much lower values than thesolar one (~2) in order to fit the observations, and if Av = 0.0 mag then 0.5 <alpha < 1. We give an equation to estimate t from M, Yi (alpha) and Av.Establishing a reference alpha = 1.00 and adopting Yi = 0.245 we derive a massand age of HD140283: M = 0.780 +/- 0.010 Msol and t = 13.7 +/- 0.7 Gyr (Av =0.0) or M = 0.805 +/- 0.010 Msol and t = 12.2 +/- 0.6 Gyr (Av=0.1 mag). Ourstellar models yield an initial metallicity of [Z/X]i = -1.70 and logg = 3.65+/- 0.03. Asteroseismic observations are critical for overcoming limitations inour results.

Benchmark stars for Gaia: fundamental properties of the Population II star HD140283 from interferometric, spectroscopic and photometric data