Field of GRB 150309A. Top panel: K_S-band discovery image (in green circle) of the afterglow of GRB 150309A taken at the GTC (+CIRCE) in 2015. The NIR afterglow is close to the centre of the image. Bottom panel: Late-time g-filter image of the field observed using the GTC in July 2016. The potential host galaxy is close to the centre of the image (within the XRT error circle of radius 1.4 arcsec, shown with a blue circle). North is in the upward direction, and east is towards the left direction.