The pre-main sequence (PMS) star ABDorA is the main component of the quadruple system ABDoradus. The precise determination of the mass and photometry of the close companion to ABDorA, ABDorC, has provided an important benchmark for calibration of theoretical evolutionary models of low-mass stars. The limiting factor to the precision of this calibration is the age of the system, as both the mass and luminosity of ABDorA and C are well monitored by other ongoing programs. In this paper we present VLTI/AMBER observations of ABDorA which provide a direct measurement of the size of this star, 0.96+/-0.06 Rsun. The latter estimate, combined with other fundamental parameters also measured for this star, allows a precise test of PMS evolutionary models using both H-R diagrams and mass-radius relationships. We have found that our radius measurement is larger than that predicted by the models, which we interpret as an evidence of the oversizing produced by the strong magnetic activity of ABDorA. Considering, at least partially, this magnetic effect, theoretical isochrones have been used to derive constraints to the age of ABDorA, favouring an age about 40-50 Myr for this system. Older ages are not completely excluded by our data.
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