just for the sake of cutting things short you may talk to the last year contract holder for the information on repurchase of these bonds acting for Petrobras:
D.F.King & Co Inc
48 Wall Street,
22nd Floor,
New York, New York 10005
USA
phone: +1- (212) 269-5550 for banks and brokers
(800) 859-8508 for all others
I believe there are so many fakes in the market and I know of some legal hurdles but its best to start asking at the sources of information before spending money on lawyers
You may talk to Bank of New York New York and Luxembourg as the depositary and to Morgan Stanley and UBS as marketmakers for Petrobras.
and next time do your DD, its just a google away and never mind ask the issuer.