Vacuum fluctuations of (q)over-barq and values of low-energy constants

We discuss the influence of the vacuum fluctuations of (q) over bar q pairs on low-energy constants and condensates. The analysis of the Goldstone bos on masses and decay constants shows that the three-flavor condensate and so me low-energy constants are very sensitive to the value of L-6, which measu res the Zweig-rule violation in the scalar channel. A chiral sum rule based on experimental data in this channel is used to constrain L-6, confirming a significant decrease between the two- and the three-flavor condensates. ( C) 2000 Published by Elsevier Science B.V.