WASHINGTON — Interest rates on short-term Treasury bills were mixed in Monday's auction, with three-month bills declining slightly and six-month bills rising.

The Treasury Department auctioned $9.5 billion in three-month bills at a discount rate of 6.090 percent. Another $8.5 billion in six-month bills was auctioned at a discount rate of 6.075 percent.

The three-month rate was down from 6.095 percent last week and was the lowest since three-month bills averaged 6.010 percent on July 24. The six-month rate was up from 6.060 percent last week and was the highest since 6.115 percent on July 31.

The new discount rates understate the actual return to investors -- 6.269 percent for three-month bills with a $10,000 bill selling for $9,846.10 and 6.354 percent for a six-month bill selling for $9,692.90.

In a separate report, the Federal Reserve said Monday that the average yield for one-year Treasury bills, the most popular index for making changes in adjustable rate mortgages, jumped to 6.17 percent last week from 6.09 percent the previous week.