Context : \alpha Centauri is our closest stellar neighbor , at a distance of only 1.3 pc , and its two main components have spectral types comparable to the Sun .
This is therefore a favorable target for an imaging search for extrasolar planets .
Moreover , indications exist that the gravitational mass of \alpha Cen B is higher than its modeled mass , the difference being consistent with a substellar companion of a few tens of Jupiter masses .

Aims : We searched for faint comoving companions to \alpha Cen B .
As a secondary objective , we built a catalogue of the detected background sources .

Methods : We used the NACO adaptive optics system of the VLT in the J , H , and K _ { s } bands to search for companions to \alpha Cen B .
This instrument allowed us to achieve a very high sensitivity to point-like sources , with a limiting magnitude of m _ { Ks } \approx 18 at 7 ” from the star .
We complemented this data set with archival coronagraphic images from the HST-ACS instrument to obtain an accurate astrometric calibration .

Results : Over the observed area , we did not detect any comoving companion to \alpha Cen B down to an absolute magnitude of 19-20 in the H and K _ { s } bands .
However , we present a catalogue of 252 background objects within about 15 ” of the star .
This catalogue fills part of the large void area that surrounds \alpha Cen in sky surveys due to the strong diffused light .
We also present a model of the diffused light as a function of angular distance for the NACO instrument , that can be used to predict the background level for bright star observations .

Conclusions : According to recent numerical models , the limiting magnitude of our search sets the maximum mass of possible companions to 20-30 times Jupiter , between 7 and 20 AU from \alpha Cen B .