Context : HR4796 is a young , early A-type star harbouring a well structured debris disk , shaped as a ring with sharp inner edges .
The inner edge might be shaped by a yet unseen planet inside the ring ; the outer one is not well understood .
The star forms together with the M-type star HR4796B , a binary system , with a projected separation of \simeq 560 AU .

Aims : Our aim is to explore the surroundings of HR4796A and B , both in terms of extended or point-like structures .

Methods : Adaptive optics images at L ’ -band were obtained with NaCo in Angular Differential Mode and with Sparse Aperture Masking ( SAM ) .
We analyse the data as well as the artefacts that can be produced by ADI reduction on an extended structure with a shape similar to that of HR4796A dust ring .
We determine constraints on the presence of companions using SAM and ADI on HR4796A , and ADI on HR4796B .
We also performed dynamical simulations of a disk of planetesimals and dust produced by collisions , perturbed by a planet located close to the disk outer edge .

Results : The disk ring around HR4796A is well resolved .
We highlight the potential effects of ADI reduction of the observed disk shape and surface brightness distribution , and side-to-side asymmetries .
We produce 2D maps of planet detection limits .
No planet is detected around the star , with masses as low as 3.5 M _ { Jup } at 0.5 ” ( 58 AU ) and less than 3 M _ { Jup } in the 0.8-1 ” range along the semi-major axis .
We exclude massive brown dwarfs at separations as close as 60 mas ( 4.5 AU ) from the star thanks to SAM data .
The detection limits obtained allow us to exclude a possible close companion to HR4796A as the origin of the offset of the ring center with respect to the star ; they also allow to put interesting constraints on the ( mass , separation ) of any planet possibly responsible for the inner disk steep edge .
Using detailed dynamical simulations , we show that a giant planet orbiting outside the ring could sharpen the disk outer edge and reproduce the STIS images published by Schneider et al .
( 2009 ) .
Finally , no planets are detected around HR4796B with limits well below 1 M _ { Jup } at 0.5 ” ( 35 AU ) .

Conclusions :