Investors raced to buy stock of Northwest Airlines and United Airlines on Thursday, betting that a takeover would bring a significant premium for their shares.

But a deal doesn't guarantee a big payday for shareholders.

A merger with Delta by either carrier instead might be done as a stock swap, in which shareholders of the two firms get stock in the new, merged firm either on a one-for-one basis or at another ratio set by the boards of directors.

In that scenario, there might be no premium for shares of Northwest or United parent UAL Corp. Instead, investors would be asked to accept the promise of bigger profits down the road from the new, bigger carrier.

"Our analysis assumes Delta acquires Northwest in a stock swap with no premium,'' UBS Securities analyst Kevin Crissy wrote in a research report this week.

The Wall Street Journal, citing anonymous sources said to be familiar with Delta's plans, reported that Delta intends to pursue its deal as a stock swap.

The possibility of a modest or no premium for Northwest shares might have been reflected in after-hours trading Thursday, where the carrier's shares fell modestly after rising more than 30 percent during regular trading.

For Northwest stockholders who bought the stock in June, when it first began trading hands after the company's bankruptcy, there's still a ways to go to make them whole.

Shares of Northwest began trading then at about $25, well above Thursday's close of $15.85.

Delta management, having just emerged from their own firm's bankruptcy last spring and now trying to deal with oil prices near $100 a barrel, could have little appetite for taking on extra debt to cash out Northwest or United investors at a fat price. And pilots at all three airlines have said they will fight any deal that threatens to keep their wages and benefits under pressure. They also are almost certain to press for an equity stake in the combined carrier.

"We can make mergers work, or we can destroy them," John Prater, head of the Air Line Pilots Association, said in December. "We are interested in mergers that form solid and profitable, long-term companies, not ones that just create a transaction so that the investors can cash out."

Even if Delta and Northwest merged as equals with no premium for the stock, the combined carrier would see a bigger percentage increase in its debt from the deal than it would in revenues, based on recent financial information from the companies.

Delta has $18.3 billion in annual revenue and Northwest $12.4 billion. Combined they also had more than $5 billion in cash on hand.

Delta's debt was $8.6 billion, while Northwest's was $6.9 billion, so a combination between the two promises two-thirds again Delta's current revenue but about twice its current debt load.

Thomas Buckingham • 612-673-7717