Stability of vertical minimal surfaces in three-dimensional sub-Riemannian manifolds

We obtain first and second variation formulae for minimal surfaces in three-dimensional sub-Riemannian manifolds which are vertical, i.e., perpendicular to the horizontal distribution of the sub-Riemannian structure. We use these formulae to explore the connection between Riemannian and sub-Riemannian properties of a surface. We also describe vertical minimal surfaces of left-invariant sub-Riemannian structures on some three-dimensional Lie groups and find out whether they are stable.