  We propose a non relativistic effective Lagrangian approach to study hadronic
atom observables in the framework of QCD (including photons). We apply our
formalism to derive a general expression for the width of the pi+pi- atom
decaying into two neutral pions. It contains all terms at leading and
next-to-leading order in isospin breaking. The result allows one to evaluate
the combination a0-a2 of pi-pi S-wave scattering lengths from pi+pi- atom
lifetime measurements, like the one presently performed by the DIRAC expeiment
at CERN.
