Activist investor Keith Meister on May 21 reunited with billionaire investor Carl Icahn and said in a regulatory filing that they may try to buy oil and gas producer Energen Corp. (NYSE: EGN).

The announcement comes roughly two months after the Birmingham, Ala.-based company settled a long-simmering fight with Meister's New York-based hedge fund Corvex Management by agreeing to review its businesses and adding two members to the board.

Now Meister and Icahn have laid out a path where they might step into the strategic review process and prepare to take over the company themselves.

"The Reporting Persons believe the issuer's securities are undervalued and may, subject to due diligence, have interest in joining with other parties to acquire the Issuer as part of the strategic initiatives process or otherwise," the filing said.

Corvex also sold 2 million shares in Energen to Carl 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% of Energen. The company's share price climbed 2.5% to $69.06 in early trading on the New York Stock Exchange on May 21.

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

In March Energen invited energy industry entrepreneur Jonathan Cohen and investor Vincent Intrieri, a longtime associate of Icahn's, to join the board.

Meister worked for Icahn for seven years and served on a number of boards where the billionaire investor owned stakes. In 2010, Meister went out of on his own and founded Corvex.