CHICAGO ( TheStreet) -- Shares in Continental ( CAL - Get Report) and United ( UAUA) were rising Monday after the Justice Department's surprisingly quick approval of their merger plans.

The carriers announced Friday after the market closed that the department's antitrust evaluation had been completed following its agreement to turn over 36 slots at Newark Airport to Southwest ( LUV - Get Report).

That deal should bring a bit of serious low-fare competition to the fortress hub with the fifth-highest average fares in the country during the first quarter, according to the Bureau of Transportation Statistics. But it should not much bother the new United, as it pushes even more passengers through the New York area's premier hub.

In a report Monday morning, Stifel Nicolaus analyst Hunter Keay called United "a compelling investment opportunity" with a chance to double, "barring a full-blown economic double-dip scenario."

Keay said United is "trading at an inexplicably significant discount" to other airlines and said management estimates of $1 billion to $1.2 billion in annual synergies "will likely prove to be conservative." He also anticipates significant "balance sheet deleveraging" once the merger is complete.

Meanwhile, Avondale Partners analyst Bob McAdoo reiterated an outperform rating on both carriers. "Buy these airlines on recent weakness as travelers are filling planes at profitable prices, economic gloom & doom notwithstanding," he said, in a report Monday morning.

In mid-morning trading, stock in Continental was up 47 cents to $22.26 while United stock was up 60 cents to $21.05.

The carriers said Friday they expect the merger to be completed Oct. 1, following shareholder approvals. The exchange ratio is roughly one-to-one, as 1.05 United shares will be exchanged for each Continental share. On April 30, the last trading day before the merger was announced, Continental stock closed at $22.35 and United stock closed at $21.60.

-- Written by Ted Reed in Charlotte, N.C. .

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