Slim, a telecommunications tycoon with U.S. assets in retail, is the second prominent investor this year to buy a piece of the publisher of The New York Times, the Boston Globe and smaller daily newspapers.

He told reporters that he bought the stake for financial reasons, indicating that he was not making a strategic move into U.S. media.

Earlier this year, hedge fund Harbinger Capital Partners, led by Phil Falcone, amassed a stake in the Times that is about equal to that of the Ochs-Sulzberger family, which controls the newspaper publisher through a special class of shares.

Harbinger had sought changes at the Times to get it to deal with its eroding share price, which has fallen as the company's revenue from advertising has dropped and readers turn to the Internet for news.

Times shares rose 92 cents, or 6.6 percent, to $14.88 in morning trading on the New York Stock Exchange.

However, Goldman Sachs analyst Peter Appert wrote in a research note that he did not think Slim's interest would be a precursor to restructuring at the Times, given the Sulzberger family's control of the company.

"More importantly ... in light of the fundamental challenges facing the company we do not believe that restructuring action (such as asset sales, a go-private transaction, etc.) would necessarily translate into a valuation meaningfully higher than the current share price," Appert wrote.

He maintains a "sell" rating on the Times shares, saying key risks are weak trends in the advertising market and the family-controlled board. "We believe NYT shares are overvalued in the context of the extraordinarily difficult newspaper operating environment and the resulting downward pressure on earnings estimates," he said.

Slim's telephone company Telmex has 90 percent of Mexico's 20 million fixed-line phones. His America Movil is Latin America's biggest mobile phone company.

His business empire includes department stores, a banking group, restaurants and manufacturers of cigarettes, floor tiles and car parts.

(Reporting by Tiffany Wu and Robert MacMillan; Editing by Steve Orlofsky)