CHARLOTTE, N.C. (

TheStreet

) -- Thursday was a good day for owners of stock in

US Airways

(LCC)

, which rose 11% after a story was leaked about the carrier's merger talks with

United

( UAUA).

But given that United has made it clear it would prefer to merge with

Continental

(CAL) - Get Report

, investors may have taken a leap of faith by jumping on the US Airways shares.

Among the United/US Airway deal's skeptics is Robert Roach, general vice president of the International Association of Machinists, the largest union at both United and US Airways. "This could be speculators pushing up the price of the stock, or it could be a ploy to get Continental to make a move on United," Roach said in an interview. "There's been a lot of integration between Continental and United," he said, noting their extensive code-share agreement and global anti-trust immunity, both of which followed United's invitation to Continental to join the Star Alliance.

Roach said he spoke Thursday with executives of Continental, who told him they were surprised to learn of the US Airways/United talks in

The New York Times

. Last month, Continental CEO Jeff Smisek said that if conditions warranted, he would resume the consolidation discussions with United that broke off two years ago. A Continental spokesman declined to comment.

Another labor skeptic is Pat Friend, president of the Association of Flight Attendants, which represents flight attendants at both carriers. "I'm not so sure this is real," she said, adding that US Airways shareholders benefitted from the story and that Continental may be forced to show its hands.

In a post Thursday on Swelblog.com, Bill Swelbar, research engineer at MIT, also questioned the merger talk. "I think the labor issues are insurmountable," he said. In the merger between

Delta

(DAL) - Get Report

and Northwest, "Delta was blessed with a pilot group that understood things needed to be in place on day one," said Swelbar. "If the template is Delta/Northwest, United/US Airways is anything but." Also, he said, a US Airways/United merger would face regulatory barriers in Washington, where US Airways dominates Reagan National airport while United dominates at Dulles.

Additionally, in July, Continental joined United and eight partners -- all members of the Star Alliance -- in a global antitrust immunity agreement. Now United has an application pending to include

ANA

in the agreement, which would allow United, Continental and ANA to form a trans-Pacific joint venture. US Airways is not included in the immunity agreement. "Why would United possibly jeopardize the international potential to merge with a U.S. domestic-oriented carrier?" Swelbar said.

In a report issued Thursday, UBS analyst Kevin Crissey noted that for United, Continental is "probably a better alternative" than US Airways. "We expect United to also speak with Continental," he wrote.

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