Mexico\'s Carlos Slim, the world\'s richest man, has acquired an 8.4 percent stake in YPF, the recently nationalized Argentine subsidiary of Spanish oil firm Repsol, according to the latest filings. Ratings agency Moody\'s had on Tuesday downgraded YPF another notch a month after Buenos Aires seized control of the country\'s largest oil firm -- it accounts for a third of domestic production -- citing concerns over its debt. A filing with the US Securities and Exchange Commission showed that Slim\'s investment company, Domitgrupo Financiero Inbursa, had acquired almost 33 million shares in YPF, which is listed on the New York Stock Exchange. Argentina declared YPF a public utility on April 16, with the government seizing a 51 percent stake in the Repsol subsidiary -- infuriating Madrid, and sparking concern in the United States and the European Union. Later the same week, it extended the move to YPF Gas, a separate company 85 percent owned by a division of Repsol. President Cristina Kirchner has argued that the expropriation was justified because YPF crude production had dropped while oil and gas imports doubled in 2011. Imports are forecast to triple by the end of the year. Slim, a 72-year-old Mexican tycoon who built his fortune in the telecoms industry, is the richest man in the world with an estimated $69 billion fortune, according to Forbes magazine.