* Brazil president Rousseff among those cleared by prosecutor
* TCU audit court opens probe of 'possible' deal irregularities
* Former Petrobras employees could have to repay $792 mln (Adds information about new federal accounts investigation, share price)
RIO DE JANEIRO, July 23 (Reuters) - Brazil's top public prosecutor cleared board members of state-run oil company Petroleo Brasileiro SA of wrongdoing in approving the 2006 purchase of an oil refinery in Pasadena, Texas, the prosecutor's office said in a statement on Wednesday.
Board members who approved the sale included chairwoman Dilma Rousseff, who is now president of Brazil.
The purchase of the refinery became the subject of criminal and congressional investigations earlier this year after politicians raised concerns about how the oil company's investment in the refinery ballooned - from $360 million in 2006 to nearly $2 billion today.
The investigations gained momentum after the arrest earlier this year of Paulo Roberto da Costa, former head of refining at Petrobras, as the oil company is known. Da Costa was arrested as part of a money-laundering probe.
While the prosecutor cleared board members, Brazil's federal accounts auditor, the TCU, said later on Tuesday that it has opened a formal probe into alleged financial irregularities in the approval of the Pasadena refinery purchase.
If any irregularities are found, board members including former Chief Executive Officer Jose Sergio Gabrielli could be ordered to repay nearly $792 million to Petrobras, the TCU said. Other current and former employees cited include Chief Financial Officer Almir Barbassa and da Costa.
While the TCU mentioned Gabrielli and other Petrobras employees as targets of the probe, Rousseff's name was not included in the documents related to the TCU case.
The prosecutor, who is based in the capital Brasilia, also instructed the federal prosecutor's office to direct any future investigations into the refinery deal to prosecutors in Rio de Janeiro, where Petrobras is based.
Petrobras preferred shares, the company's most-traded class of stock, fell 3.47 percent to 20.32 reais in Sao Paulo trading on Wednesday, their first decline in 11 days and biggest one-day drop in a month. (Reporting by Jeb Blount; editing by Matthew Lewis and Gunna Dickson)