SAO PAULO (Dow Jones)--The Brazilian central bank on Tuesday sold the equivalent of $848.2 million in reverse swap contracts on offer at a surprise auction, the bank said.

The central bank sold 86.2% of the 20,000 contracts offered with three different maturities, two being due in 2011 and one in 2012, the bank said on its website.
It was the eighth reverse swap auction held by the central bank so far this year. At a reverse swap auction on Feb. 18, the central bank sold 86% of the contracts offered, worth $1.48 billion.
Reverse swap auctions offer investors the opportunity to exchange futures positions in U.S. dollars for positions tied to domestic interest rates.
Typically, a reverse swap auction supports the dollar against the Brazilian real by removing future dollar deliveries from the market. The government revived reverse swap auctions earlier this year in an effort to arrest the continued appreciation of the Brazilian real against the dollar.
The real has gained more than 30% against the dollar since the beginning of 2009. The strong real is hurting Brazilian exports.