A teat cup shell (2) includes a substantially tubular body having a center axis (4). A plane (6) extending along the center axis (4) defines a first shell portion (8) and a second shell portion (10) on opposite sides of the plane (6). The first shell portion (8) includes a magnetic material, and an outer surface (12) adapted to abut against at least one surface of an electromagnetic device. The first shell portion (8) and the second shell portion (10) differ from each other with respect to material composition and/or material thickness. The amount of magnetic material in the first shell portion (8) is higher than in the second shell portion (10). Further, a teat cup and an arrangement for automatically milking animals includes at least one robot arm (32) adapted to attach the teat cup (16) to a teat of an animal.