(Rewrites throughout, adding background and comments from a stock analyst)

 
  By Taos Turner

BUENOS AIRES--Mexican billionaire Carlos Slim has bought an 8.4% stake in YPF SA (YPF, YPFD.BA), Argentina's biggest oil-and-gas company.

Mr. Slim acquired 6.59% of YPF's stock, or almost 25.9 million Class D shares, for $288 million, according to a filing YPF sent to the local stock exchange Thursday. He also acquired another 1.77%, or almost seven million shares, as part of the deal.

The Mexican investor obtained the stake through Grupo Financiero Inbursa SAB de CV (GPFOY, GFINBUR.MX).

The shares previously belonged to Argentina's Eskenazi family, which owned 25.46% of YPF through Petersen Energia SA.

The family lost control of those shares as a result of the Argentine government's expropriation last month of 51% of YPF from its majority shareholder, Spain's Repsol YPF SA (REP.MC, REPYY). The nationalization forced Petersen Energia to give up all of its shares as part of a previous deal with banks, which had helped lend the family enough money to buy the stocks.

YPF's shares in New York jumped 8.1% in after-hours trading Thursday. During Thursday's session, the stock closed up 1.9% at $10.45.

"Obviously, this will have a temporary impact on the price," said Juan Jose Vazquez, an analyst at Bull Market Brokers in Buenos Aires. "The market's concern is related to YPF's management and how much liberty it will have. But if someone like Slim has invested, it's a good sign."

Government officials said it was necessary to nationalize YPF to boost declining oil and gas production in Argentina. Argentine President Cristina Kirchner accused Repsol of under-investing in output, a charge the Spanish company vigorously denied.

Ms. Kirchner said YPF would have twin goals of boosting output while also helping the country meet its broader objective of offering cheap oil and gas to individuals and companies.

However, analysts and industry executives say these goals are incompatible and they question how much autonomy YPF's new management will have given pressure to keep prices low.

YPF recently announced plans to invest $35 billion to raise oil and gas output, but it's unclear where that money will come from. Earlier this week, Moody's Investors Service downgraded YPF one notch further into junk territory, highlighting the company's debt challenges.

The stock-exchange filing said Mr. Slim has no plans to acquire more shares or obtain more control over YPF.

Mr. Slim's press office declined to comment.

Mr. Slim is the world's richest man, with a net value of around $69 billion, according to a ranking by Forbes magazine.

He already has a significant investment in Argentina through America Movil (AMX), one of the country's biggest cellphone companies.

The billionaire has a long history of buying stakes in companies he sees as undervalued, including the Mexican construction firm Empresas ICA (ICA). He previously invested in the Latin American oil and gas industry through Grupo Carso SAB, which bought a 70% stake in the Tabasco Oil Company.

Laurence Iliff in Mexico City contributed to this article.

Write to Taos Turner at taos.turner@dowjones.com