American Airlines and British Airways offered to give up four takeoff and landing slots in London.

The airlines, which want to form a trans-Atlantic joint venture that includes the Spanish carrier Iberia, told the commission that they will make slots available to other carriers on routes between London’s Heathrow and Gatwick airports to Boston, New York, Miami and D/FW.

The venture would also provide access to frequent flier programs on the relevant routes, which would allow “passengers of the qualified new entrants to accrue and redeem miles on the parties’ frequent flier programs.”

If the request is approved, American, British Airways and Iberia could cooperate on scheduling, marketing and other business functions on trans-Atlantic routes. Antitrust immunity also lets them share expenses and revenue.

The commission said it has invited comments from interested parties, such as competitor Virgin Atlantic. Richard Branson, CEO of Virgin Atlantic, said the slot concessions are “woefully inadequate in counteracting the anti-competitive harm of a combined BA/AA.”

Last month, American and British Airways received conditional approval of their antitrust immunity application from the U.S. Transportation Department if the airlines give up four slots, including two slots between London and Boston. The other two slots had yet to be determined.