Mem. S.A.It. Vol. 84, 1044 c  SAIt 2013 Memorie della ⋆ J. C. Guirado 1 , J. M. Marcaide 1 , J.-P. Berger 2 , A. M´ erand 2 , and I. Mart´ ı-Vidal 3 1 Dpartamento de Astronom´ ıa y Astrof´ ısica, Universidad de Valencia, Dr. Moliner 50, 46100, Burjassot, Valencia, Spain, e-mail: guirado@uv.es 2 European Southern Observatory, Alonso de Cordova, 3107, Vitacura, Chile 3 Onsala Space Observatory, SE-43992 Onsala, Sweden Abstract. The precise determination of both the dynamical mass and infrared photom- etry of the close companion to AB Dor A, AB Dor C (0.090 M ⊙ ), has provided an impor- tant benchmark for calibration of theoretical evolutionary models of low-mass young stars. However, comparison of the observed magnitudes of AB Dor C with these models suggests that they could overpredict the flux of this object, a trend also found in other young systems. One of the ambiguities remaining in AB Dor C is the possible binary nature of this star; in fact, should AB Dor C be close binary (∼10 mas separation), it could reconcile observations and models. We will report on VLTI/AMBER observations of AB Dor C addressed to dis- criminate between both scenarios: AB Dor C as a single object or a binary brown dwarf. We used a non-standard “off-axis” fringe tracking that allowed the detection of AB Dor C. This is, to our knowledge, one of the weakest object detected by an infrared interferometer (K s = 9.5). Key words. stars: low-mass stars – techniques: interferometry – stars: individual: AB Doradus C 1. Introduction Studies of fundamental parameters of very low-mass objects are relevant since they pro- vide tests of stellar evolution models that are used to derive the theoretical masses of brown dwarfs and planets. Only a handful of stellar systems, those with dynamically-determined masses, can effectively evaluate the predictions of the models. One of these systems is the pair AB Dor A (K s = 4.686) and the low-mass Send offprint requests to: J. C. Guirado ⋆ Based on observations made with ESO Telescopes at the Paranal Observatory under programme ID 090.C-0559(A). companion AB Dor C (K s = 9.5; Close et al. 2005, 2007). Independent measurements of the infrared photometry and the dynamical mass of AB Dor C make this object appropriate to calibrate mass-luminosity relations. Actually, comparison of observed magnitudes with theo- retical mass-luminosity relationships (DUSTY models; Chabrier et al. 2000) suggests that the models tend to underpredict the mass of AB Dor C, or equivalently, overpredict the flux of the object, specially at J and H bands (Close et al. 2007). A similar trend has been found in other young binary systems (Reiners et al. 2005). Given the need for an accurate calibra- tion of the models, any possible ambiguity or