Quasi-minimal Rotational Surfaces in Pseudo-Euclidean Four-dimensional   Space
  In the four-dimensional pseudo-Euclidean space with neutral metric there are three types of rotational surfaces with two-dimensional axis - rotational surfaces of elliptic, hyperbolic or parabolic type. A surface whose mean curvature vector field is lightlike is said to be quasi-minimal. In this paper we classify all quasi-minimal rotational surfaces of elliptic, hyperbolic and parabolic type, respectively. 
Georgi Ganchev and Velichka Milousheva
math.DG