Non-confirmation of reported HI clouds without optical counterparts in   the Hercules Cluster
  21 cm HI line observations were made with the Arecibo Gregorian telescope of 9 HI clouds in the Hercules Cluster which were reported as tenative detections in a VLA HI study of the cluster (Dickey 1997) and for which our deep CCD imaging failed to find any optical counterparts. No sensitive observations could be made of one of these (sw-174) due to the presence of a close-by strong continuum source. The other 8 tentative HI detections were not reconfirmed by the Arecibo HI measurements. The CCD images did reveal faint, low surface brightness counterparts near the centres of two other VLA HI sources invisible on the Palomar Sky Survey, sw-103 and sw-194. 
W. van Driel, K. O'Neil, V. Cayatte, P.-A. Duc, J.M. Dickey, C.   Balkowski, H. Hernandez, J. Iglesias-Paramo, P. Papaderos,...