Comment on "Generating Unexpected Spin Echoes in Dipolar Solids with Pi   Pulses" arXiv:0705.0667
  The present work is a comment on PRL 98, 190401 (2007), arXiv:0705.0667 . The authors reported NMR measurements in solids with different pulse sequences. In some multiple $\pi$ pulse echo trains they observed coherent signals of several seconds, well beyond the Hahn echo decay, $T_{2HE}$ (few milliseconds). They ascribed the long tails to coherent phenomena arising on the dipolar couplings during any real finite NMR pulse. Their statement is based on some assumptions that are incorrect and parameters in the calculations, that do not represent the experimental conditions. 
Patricia R. Levstein, M. Belen Franzoni