Odd and Even Partial Waves of $\eta\pi^-$ and $\eta'\pi^-$ in 191 GeV/c $\pi^{-}p$

In the year 2008 COMPASS recorded diffractive events of the signature \pi^-(191 GeV) p --> X_fast p. We present results of the analysis of the subsystems X = \eta(')\pi^-. Besides the known resonances a_2(1320), a_4(2040), we study the properties of the spin-exotic P+ wave, and all other natural-exchange partial waves up to spin J = 6. We find a striking difference between the two final states: whereas the even partial waves J = 2, 4, 6 in the two systems are related by phase-space factors, the odd partial waves are relatively suppressed in the \eta\pi^- system. The relative phases between the even waves appear identical whereas the phase between the D and P waves behave quite differently, suggesting different resonant and non-resonant contributions in the two odd-angular-momentum systems. Branching ratios and parameters of the well-known resonances a_2 and a_4 are measured.