Ellipsoidal shapes in general relativity: general definitions and an application

A generalization of the notion of ellipsoids to curved Riemannian spaces is given and the possibility of using it in describing the shapes of rotating bodies in general relativity is examined. As an illustrative example, stationary, axisymmetric perfectfluid spacetimes with a so-called confocal inside ellipsoidal symmetry are investigated in detail under the assumption that the 4-velocity of the fluid is parallel to a timelike Killing vector field. A class of perfect-fluid metrics representing interior NUTspacetimes is obtained along with a vacuum solution with a non-zero cosmological constant.