Ca II H and K index plotted vs. V magnitude for the M 2 giants. A probable non-member star is shown as an open square. The small scatter, fully compatible with the formal measurement errors, in the Ca(HK) values as a function of V provides additional evidence that all spectroscopic targets but the notable outlier are members of M 2. The open red symbol refers to star 10427, which is not a member of M 2, according to its radial velocity.