In this project, we are interested in probing the magnetic phase diagram of the diluted XY pyrochlore antiferromagnet Er2Ti2O7. Theoretical works have predicted that dilution should induce a transition from a pure psi2 state to a pure psi3 state at a critical dilution concentration of ~10%. To verify such theoretical prediction, we grew single crystals of Er2-xYxTi2O7 (x=0,0.1,0.2,0.4) and we propose to use a very detailed polarization analysis which is required to distinguish a pure psi2 and pure psi3 state. Such highly sophisticated experiment is possible to perform at ILL using the CRYOPAD setup on the D3 diffractometer. The discovery of a magnetic transition from a pure psi2 to a pure psi3 state is very appealing as it would be driven by order-by-disorder which is a phenomenon that selects long-range order via fluctuations (entropic selection) instead of a usual energetic selection.