The cosmological constant and the energy of gravitational radiation

We propose a definition of mass for characteristic hypersurfaces in asymptotically vacuum space-times with nonvanishing cosmological constant Lambda is an element of R*, generalizing the definition of Trautman and Bondi for Lambda = 0. We show that our definition reduces to some standard definitions in several situations. We establish a balance formula linking the characteristic mass and a suitably defined renormalized volume of the null hypersurface, generalizing the positivity identity proved by Chrusciel and Paetz when Lambda = 0.