WASHINGTON — Interest rates on short-term Treasury bills were mixed in Monday’s auction, with rates on six-month bills declining to their lowest since early August and three-month bills unchanged.

The Treasury Department auctioned $32 billion in three-month bills at a discount rate of 0.105 percent, unchanged from last week. Another $28 billion in six-month bills was auctioned at a discount rate of 0.140 percent, down from 0.145 percent last week.

The six-month rate was the lowest since these bills averaged 0.135 percent on Aug. 6.

For a $10,000 bill, the three-month price was $9,997.34, while a six-month bill sold for $9,992.92. That would equal an annualized rate of 0.106 percent for the three-month bills and 0.142 percent for the six-month bills.

Separately, the Federal Reserve said Monday that the average yield for one-year Treasury bills rose to 0.19 percent last week from 0.18 percent the previous week.