The U.S. participation may have been an effort to enable the Japanese to smooth gyrations in their currency without creating more market pressure for Treasury bonds.

The US Treasury informed a number of banks that it may intervene in the Japanese yen market on Friday and that they should "stand ready for future action."

July 31 : The U.S. Treasury intervened to support the yen on Friday through outright purchases, the Financial Times reported, joining Japan in its efforts to boost the currency…

Japan's finance ministry confirmed that Tokyo and Washington jointly intervened in currency markets to support the yen for the first time since 2011. The coordinated move…