We are grateful to Ken Libbrecht and Martin Woodard for providing us their fine data over the years. We are also grateful to Yvonne Elsworth for the numbers in Table 2. This work was supported by AFOSR-95-0070, NSF-INT-93-14820, NSF-AST-96-14803, and by KBN-2P304-013-07.
© European Southern Observatory (ESO) 1997
Online publication: July 8, 1998