Positivity of the Bondi Energy in Einstein-Maxwell Axion- Dilaton   Gravity
  We present a proof of the positivity of the Bondi energy in Einstein-Maxwell axion-dilaton gravity, being the low-energy limit of the heterotic string theory. We consider the spacelike hypersurface which asymptotically approaches a null cone and on which the equations of the theory under consideration are given. Next, we generalize the proof allowing the hypersurface having inner boundaries. 
Marek Rogatko (Tech. Un. of Lublin)