Take the hydrogen atom. It is easy to imagine that the gravitational pull it creates is smaller than the sum of those of the proton plus the electron, because a photon of 13.6 eV was created when the atom assembled, and has left the system. In other words, the binding energy between the proton and the electron has to be subtracted to the mass of the system.
But where does this appear in general relativity? How does it fit in the stress-energy tensor? I know how to define this tensor for one particle, or for an electromagnetic field, but I don't see how the binding energy fits.