A fin (4, 4a, 4b, 4c, 4d) for a boiler tube arrangement comprising a number of boiler tubes (34, 36, 38, 40) extending along each other, and an assembly (32) comprising such a boiler tube arrangement and such a fin, are provided. A longitudinal center axis (C) of the fin is arranged to extend perpendicular to a length of the boiler tubes, and the fin comprises an inner edge (8) and an outer edge (10). The fin is characterized in that a contour of the inner edge is essentially identical to a contour of the outer edge. Each of the contours of the inner and outer edges definies a number of ridges (16, 18, 20, 26, 28, 30) and a number of valleys (12, 14, 22, 24), which ridges and valleys are alternately arranged and connected to each other at transitions (P, P') being half way between a highest point (H, H') and a lowest point (L, L') of the connected ridge and valley, respectively. The valleys defined by the inner edge are each arranged to receive, and only partly enclose, a respective one of the boiler tubes.