* Ex-president takes on parliament speaker, opposition boss
* Island has not yet found oil in its share Gulf of Guinea
By Ricardo Neto
SAO TOME, July 3 (Reuters) - Candidates for the presidency of Sao Tome and Principe launched their campaigns on Sunday for an election that will decide who runs the island nation that is struggling to win investment from oil majors.
The July 17 poll will pit former president Manuel Pinto da Costa against the current parliament speaker Evaristo de Carvalho, opposition LSTP-PSD leader Aurelio Martinez and six others.
Sao Tome and Principe is so far failing to convince oil companies to invest in its energy sector after initial excitement over the Gulf of Guinea nation faded because of disappointing results from wells.
Nigeria has a joint oil zone with Sao Tome but Angola is increasingly involved, with state-owned oil company Sonangol planning to invest more than $12 million this year to modernise the port and airport.
The government is also keen to develop tourism on the island characterised by largely unspoilt, palm fringed beaches.
The streets were festooned with campaign posters of the nine candidates, who were out holding rallies. Opinion polls published in local media put da Costa and de Carvalho ahead.
Da Costa, 74, an economist, was the former Portuguese colony’s president from independence in 1975 until 1991, running a socialist one party state in the country, which lies in the oil-rich Gulf of Guinea but has not found oil in its slice.
The former president, who was defeated in the 1996 and 2001 elections, has a close relationship with President Jose Eduardo dos Santos of oil-producer Angola, also an ex-Portuguese colony, who is likely to back him.
President Fradique de Menezes is nearing the end of his second five-year term so he cannot stand.
Election commission president Victor Correia said campaigning for the votes of the island's 92,638 registered voters would continue until July 15. (Writing by Tim Cocks; Editing by Alison Williams) (For more Reuters Africa coverage and to have your say on the top issues, visit: af.reuters.com/)