  We study the influence of off-resonant two photon transitions on high
precision measurements with atom interferometers based on stimulated Raman
transitions. These resonances induce a two photon light shift on the resonant
Raman condition. The impact of this effect is investigated in two highly
sensitive experiments: a gravimeter and a gyroscope-accelerometer. We show that
it can lead to significant systematic phase shifts, which have to be taken into
account in order to achieve best performances in term of accuracy and
stability.
