* Analysis is not a formal investigation yet - source
* PT bought 897 mln euros in Rioforte debt, due this month
* Rioforte parent ESI has already missed some debt repayments
* PT's Brazilian partner Oi seeking to protect its shareholders
LISBON, July 10 (Reuters) - Portugal's securities market watchdog, CMVM, is analysing a major investment by Portugal Telecom in the debt of Rioforte, a holding company of the Espirito Santo banking family whose companies are under regulator scrutiny, sources said.
One source familiar with the situation told Reuters that CMVM wanted to find out whether any rules had been breached by any of the parties involved, in particular whether PT obeyed disclosure rules in the 897 million euro ($1.2 billion) deal, that could affect its own financial situation.
Another source said the regulator's analysis was not yet a formal investigation.
CMVM officials declined to comment. Nobody was available to comment at Portugal Telecom, which has previously defended the investment as based on a long history of such investments with the Espirito Santo group and offering a good rate of return.
PT last month said it had invested 897 million euros in commercial paper issued by Rioforte, which falls due on July 15 and 17. Its shares have tumbled on fears it may not get the money back after auditors revealed financial irregularities at Rioforte's parent company, Espirito Santo International.
ESI has already failed to reimburse at least some debt holders on maturity, according to Banque Privee Espirito Santo.
Rioforte's unit Espirito Santo Financial Group (ESFG) is the largest shareholder in Banco Espirito Santo (BES) which in its turn is the largest single shareholder in Portugal Telecom, with a 10 percent stake. PT also owns a stake in BES.
ESFG said on Thursday it had decided to suspend trading in its shares and bonds, citing "material difficulties" at its own parent company, in news that hit European stock and bond markets alike on Thursday.
PT is in the process of merging assets with Brazil's telecoms company Oi, whose shares have also been punished. Its management has vowed to protect shareholders and seek a solution.
Analysts say Oi may seek to revise the terms of the merger with PT if Rioforte does not pay up, and PT shareholders may end up with a much smaller stake in the combined company.
PT shares fell 4.5 percent on Thursday to 1.91 euros, near their all-time lows of 1.89 euros. BES shares plunged more than 17 percent before trading in the issue was suspended. ($1 = 0.7331 Euros) (Reporting by Sergio Goncalves; Writing by Andrei Khalip; Editing by Axel Bugge and Hugh Lawson)