Free boundary minimal annuli immersed in the unit ball

We construct a family of compact free boundary minimal annuli immersed in the unit ball ${\mathbb{B}}^3$ of ${\mathbb{R}}^3$, the first such examples other than the critical catenoid. This solves a problem formulated by Nitsche in 1985. These annuli are symmetric with respect to two orthogonal planes and a finite group of rotations around an axis, and are foliated by spherical curvature lines. We show that the only free boundary minimal annulus embedded in ${\mathbb{B}}^3$ foliated by spherical curvature lines is the critical catenoid; in particular, the minimal annuli that we construct are not embedded. On the other hand, we also construct families of non-rotational compact embedded capillary minimal annuli in ${\mathbb{B}}^3$. Their existence solves in the negative a problem proposed by Wente in 1995.