Context : Tight binaries discovered in young , nearby associations , with known distances , are ideal targets to provide dynamical mass measurements to test the physics of evolutionary models at young ages and very low masses .

Aims : We report for the first time the binarity of TWA22 .
We aim at monitoring the orbit of this young and tight system to determine its total dynamical mass using an accurate distance determination .
We also intend to characterize the physical properties ( luminosity , effective temperature and surface gravity ) of each component based on near-infrared photometric and spectroscopic observations .

Methods : We use the adaptive optics assisted imager NACO to resolve the components , to monitor the complete orbit and to obtain the relative near infrared photometry of TWA22 AB .
The adaptive optics assisted integral field spectrometer SINFONI was also used to obtain medium resolution ( R _ { \lambda } = 1500 - 2000 ) spectra in JHK bands .
Comparison with empirical and synthetic librairies were necessary to derive the spectral type , the effective temperature and the surface gravity for each component of the system .

Results : Based on an accurate trigonometric distance ( 17.53 \pm 0.21 pc ) determination , we infer a total dynamical mass of 220 \pm 21 M _ { Jup } for the system .
From the complete set of spectra , we find an effective temperature T _ { eff } = 2900 ^ { +200 } _ { -200 } K for TWA22 A and T _ { eff } = 2900 ^ { +200 } _ { -100 } K for TWA22 B and surface gravities between 4.0 and 5.5 dex .
From our photometry and a M6 \pm 1 spectral type for both components , we find luminosities of log ( L/L _ { \odot } ) =-2.11 \pm 0.13 dex and log ( L/L _ { \odot } ) =-2.30 \pm 0.16 dex for TWA22 A and B respectively .
By comparing these parameters with evolutionary models , we question the age and the multiplicity of this system .
We also discuss a possible underestimation of the mass predicted by evolutionary models for young stars close to the substellar boundary .

Conclusions :