Isabel dos Santos, the eldest daughter of Angolan President Jose Eduardo do Santos, is Africa's first woman billionaire, according to research by American financial magazine Forbes. Isabel dos Santos has bought shares in several Portuguese companies in recent years, including a bank and cable television firm. "Those stakes, combined with assets Isabel dos Santos owns in at least one bank in Angola, have pushed her net worth over the $1 billion mark ... making the 40-year-old Africa’s first woman billionaire," the magazine reported on its website late on Wednesday. Dos Santos is the main shareholder in ZON Multimedia, Portugal's largest cable television company. She owns 28.8 percent through two other companies, Kento and Jadeium, according to Forbes' research. Her 19.5-percent stake in Portuguese bank BPI, one of the largest trading on the country's stock exchange, is worth $465 million. Dos Santos also owns a quarter of Angolan bank BIC, estimated at $160 million, and a quarter of local telecommunications firm Unitel. The Unitel stake alone is worth over $1 billion, analysts told the magazine. Isabel was born in 1973 to President Dos Santos' first wife, Tatiana Kukanova, an Azeri. The two have since separated. Dos Santos became the oil-rich country's president in 1979 and today is Africa's second-longest ruling leader after Equatorial Guinea's Teodoro Obiang Nguema, who beats him by a month. Angola is the second-biggest oil producer in Africa, but its people are among the world's poorest. Most survive on less than two dollars a day. The eldest Dos Santos daughter grew up in London, where she studied mechanical and electrical engineering at King's College. Her first business venture was a restaurant, Miami Beach, which she opened as 24-year-old in Luanda. She speaks several languages and is married to Congolese art collector Sindika Dokolo. Discreet and media shy, she rarely appears in public.