I'm curious about the isospin of a photon. I have been told it is a linear combination of I=1 and I=0. If I=0, then I3 must be 0, but if I=1, what values of I3 are allowed? I don't really have a good idea of how to think about the isospin of this boson because I can't break it down into quarks like I can for pions. Any clarifications are appreciated.