SAO PAULO, Nov 25 (Reuters) - Brazil officials are worried a tumbling U.S. dollar is luring investors into risky currency derivatives contracts a year after the country grappled with massive losses from wrong, one-way wagers on the real (BRBY), O Estado de S. Paulo reported on Wednesday, citing a finance ministry aide.

Emilio Garofalo, who this month was hired to advise Finance Minister Guido Mantega on currency markets, said there are rumors that speculative transactions involving the use of currency derivatives have returned, Estado said.

Nothing against derivatives, but too much speculation involving them caused a lot of problems last year, Garofalo was quoted by Estado as saying.

Garofalo will help the ministry update foreign exchange market rules, Estado said. He defended a floating exchange rate and the updating of some currency laws that no one knows why we have today, Estado said.

The strong real between 2004 and mid-2008 allowed companies to borrow from banks using risky derivatives structures that helped cut their cost of credit as long as the currency kept strengthening against the dollar.

But the situation reversed in September 2008, following the collapse of U.S. investment bank Lehman Brothers, when a global credit crunch led to a sharp slump in the real, which fell 24 percent in 2008.

The real, the world's highest-gaining currency between 2003 and 2007, has risen 35 percent so far this year.

This month the central bank ordered non-listed companies and banks to register derivatives contracts tied to corporate loans at a clearing house in a bid to avert excess currency hedging by firms with no apparent reason for doing so.

($1=1.727 reais) (Reporting by Guillermo Parra-Bernal; Editing by Padraic Cassidy) ((guillermo.parra@thomsonreuters.com; +55-11-5644-7714; Reuters Messaging: guillermo.parra.reuters.com@reuters.net))

Lehman brothers