Photoelectron spectra of cold (10 K) size selected water cluster anions (H(2)O)(n) (-) and (D(2)O)(n) (-) have been measured in the size range n=20-120. A new isomer with a higher binding energy than the so-called isomer I has been identified, which appears in the size range n=25-30 and for (H(2)O)(n) (-) becomes dominant at n=46. Magic numbers observed in the mass spectra of the cluster anions provide evidence that this new isomer class consists of clusters with an internal electron.