The pre-main sequence ( PMS ) star AB Dor A is the main component of the quadruple system AB Doradus .
The precise determination of the mass and photometry of the close companion to AB Dor A , AB Dor C , 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 AB Dor A and C are well monitored by other ongoing programs .
In this paper we present VLTI/AMBER observations of AB Dor A which provide a direct measurement of the size of this star , 0.96 \pm 0.06 R _ { \odot } .
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 AB Dor A .
Considering , at least partially , this magnetic effect , theoretical isochrones have been used to derive constraints to the age of AB Dor A , favouring an age about 40-50 Myr for this system .
Older ages are not completely excluded by our data .