The ROSAT project has been supported by the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (BMBF) and the Max-Planck-Gesellschaft (MPG). The continued support for this project by my colleagues from the MPE ROSAT team is gratefully acknowledged. This research has also made use of the Simbad database, operated at CDS, Strasbourg, France. I acknowledge discussions with H. Hudson on solar coronal holes and with M. Hünsch on minimum fluxes, and the help of Prof. Dr. J. Trümper in organizing this paper.
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998