Left: astrometric positions of β Pic b relative from A used in the present work to characterize the orbital parameters of the β Pic b planet. The predictions for the β Pic b in case of a background source are reported in gray from November 10, 2003 to March 26, 2011. Two linear fits, passing through the origin (Δα,Δδ) = (0,0), for the 2003 NE data points (at 34.25 deg) and the 2009, 2010 and 2011 SW measurements (at 211.15 deg) are indicated by the two blue dashed lines. A zoomed view of the most recent astrometric observations over 2010 and 2011 is presented on the right.