Non-confirmation of reported clouds without optical counterparts in the Hercules clusterW. van Driel, K. O'Neil, V. Cayatte, P.-A. Duc, J. M. Dickey, C. Balkowski, H. Hernández, J. Iglesias-Páramo, P. Papaderos, J. M. Vílchez and T. X. ThuanA&A, 399 2 (2003) 433-439DOI: https://doi.org/10.1051/0004-6361:20021684