Illustration of the window used in the search for background stars. The thick solid blue line indicates the proper motion of the lens (red star), and the origin is set to the J2015.5 position of the lens. The blue dashed line indicates the real motion, which includes the parallax (only five years are shown). When a background star (yellow star) is within the black box, it is considered as candidate. For the plot, the box is defined by the position of the lens in J2010.0 and J2015.5 and a half width of 7 arcsec. To account for the proper motion of the background source, the widening shape (dotted black line) would be more physically accurate.