Activist hedge fund manager Keith Meister and billionaire investor Carl Icahn said they may try to buy oil and gas producer Energen Corp, according to a regulatory filing made on Monday.

Icahn, 82, reunited with Meister, his 45-year old former right-hand man, by buying a stake in Energen and throwing his weight behind Meister's year-long campaign to push for Energen's management to look at alternatives.

In Monday's filing, Meister's Corvex Management said Icahn bought 2 million shares and has an option to buy more. He also laid out their possible plan, saying they have "an interest in joining with other parties to acquire the Issuer."

Icahn is worth $18.3 billion, according to Forbes, putting him in a strong position to threaten and follow through on buying a company.

Two months ago, Birmingham, Alabama-based Energen, which has operations in the Permian Basin, seemed to have settled with Meister's New York-based hedge fund by agreeing to review its businesses and adding two members to the board.

In an interview with CNBC, Icahn, however, questioned why the process had not moved more quickly. "You can get a man to the moon quicker," he said.

Corvex, which invests roughly $4.4 billion, sold 2 million shares in Energen to Icahn for $64.84 per share and granted him an option to buy another 2 million shares for $67.37 per share. Icahn may convert the shares any time between now and November 18, 2018, the filing says.

Corvex owns 8.5 million shares, or roughly 8.7 percent of Energen. Together, Icahn and Meister now control roughly 10 percent of the company.

News that Icahn is involved helped push Energen's share price nearly 6 percent higher to $71.33 on the New York Stock Exchange on Monday.

Since January, the company's stock price has climbed 19.13 percent.

Meister made his first overture to Energen a year ago with a filing in late May, 2017. In March, Energen invited energy industry entrepreneur Jonathan Cohen and investor Vincent Intrieri, a longtime associate of Icahn's, to join its board, and promised a review.

Meister worked for Icahn for seven years and served on a number of boards of companies in which Icahn owned stakes. In 2010, Meister founded Corvex.