** Shares in Portugal Telecom slump 13 pct to historic low after slump in stock of Brazilian partner Oi after the departure of chief executive Zeinal Bava.
** Bava joined Oi in June 2013 with the goal of pushing through a merger with Portugal Telecom.
** Deal came under pressure after a holding company of Portugal's Espirito Santo banking family defaulted on nearly 900 mln euros it owed to Portugal Telecom, forcing a revamp of the merger deal.
** Shares in Oi fell 10.6 percent on Thursday.
** "We are on a carousel, a vicious cycle between the stocks of the companies. The intrinsic correlation between Oi and Portugal Telecom has led to this spiral of declines in PT to historic lows," said Luis Goncalves, a trader at brokerage GoBulling in Porto.
** Bava resigned after reports that billionaire Patrick Drahi's telecoms firm Altice SA was in talks to buy Oi's Portuguese assets, which became part of Oi after the merger.
** Stock down 55 percent YTD. (Reuters Messaging: email@example.com)