Context :

Aims : The aim of the project is to characterise both components of the nearest brown dwarf sytem to the Sun , WISE J104915.57 - 531906.1 ( also proposed as Luhman 16AB ) at optical and near-infrared wavelengths .

Methods : We obtained high signal-to-noise intermediate-resolution ( R \sim 6000–11000 ) optical ( 600–1000 nm ) and near-infrared ( 1000–2480nm ) spectra of each component of Luhman 16AB , the closest brown dwarf binary to the Sun , with the X-Shooter instrument on the Very Large Telescope ( VLT ) .

Results : We classify the primary and secondary of the Luhman 16 system as L6–L7.5 and T0 \pm 1 , respectively , in agreement with previous measurements published in the literature .
We present measurements of the lithium pseudo-equivalent widths , which appears of similar strength on both components ( 8.2 \pm 1.0Å and 8.4 \pm 1.5Å for the L and T components , respectively ) .
The presence of lithium ( ^ { 7 } Li ) in both components imply masses below 0.06 M _ { \odot } while comparison with models suggests lower limits of 0.04 M _ { \odot } .
The detection of lithium in the T component is the first of its kind .
Similarly , we assess the strength of other alkali lines ( e.g .
pseudo-equivalent widths of 6–7Å for Rb I and 4–7Å for Cs I ) present in the optical and near-infrared regions and compare with estimates for L and T dwarfs .
We also derive effective temperatures and luminosities of each component of the binary : - 4.66 \pm 0.08 dex and 1305 ^ { +180 } _ { -135 } for the L dwarf and - 4.68 \pm 0.13 dex and 1320 ^ { +185 } _ { -135 } for the T dwarf , respectively .
Using our radial velocity determinations , the binary does not appear to belong to any of the well-known moving group .
Our preliminary theoretical analysis of the optical and J -band spectra indicates that the L- and T-type spectra can be reproduced with a single temperature and gravity but different relative chemical abundances which impact strongly the spectral energy distribution of L/T transition objects .

Conclusions :