Minimal surfaces with closed curvature lines

We investigate complete non-orientable minimal surfaces of finite total curvature in ${\mathbb{R}}^3$ such that their ends are foliated by closed lines of curvature. This condition on the ends is necessary if they have a piece inside some Euclidean ball that is free boundary. It turns out this is a rigid situation, and we are able to show, among further obstructions, that there are no such surfaces with one end.