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Dec 21 (Reuters) - Western Refining Inc said it had reached an agreement to buy the shares of Northern Tier Energy LP it did not already own, in a deal that values the refiner at $2.43 billion.

Northern Tier unitholders will now receive $15 in cash and 0.2986 of Western Refining share for each unit held, the companies said on Monday.

Western Refining, which currently owns about 38 percent of Northern Tier, had in October offered $17.50 in cash and 0.2266 of its shares.

The revised offer values Northern Tier at $26.20 per share and represents a premium of nearly 8 percent to the stock’s closing price on Monday, according to Reuters’ calculations.

The October offer had valued Northern Tier at $27.11 per share, a premium of 12 percent to the stock’s last close.

Northern Tier unitholders are expected to own about 15 percent of Western Refining after the deal closes, expected in the first half of 2016.

Northern Tier Chief Executive Dave Lamp will be named financial officer at Western Refining once the deal closes, the companies said.

Goldman Sachs is the financial adviser and Vinson & Elkins, Davis Polk & Wardwell and Richards Layton & Finger legal advisers to Western Refining.

Evercore is the financial adviser to Northern Tier, while Akin Gump Strauss Hauer & Feld LLP and Morris, Nichols, Arsht & Tunnell LLP provided legal counsel. (Reporting by Sneha Banerjee in Bengaluru; Editing by Sriraj Kalluvila)