Upper row: Cutout of 0.9 × 0.7 sq. degrees of the galaxy-subtracted image in the r-band. Light-blue contours are the isodensity contours extracted from the surface density map (see Fig. 9, left panel). Magenta and white triangles represent the center of NGC 3640 and NGC 3641, respectively. Green labels indicate the connection regions between GC population and LSB features described in Sect. 4.1. Label A points toward an LSB tail southeast of NGC3640; label B points to shell structures northwest of NGC3640. Lower row: Zoom-in on the three labeled regions.