Delta Air Lines Inc. is buying Singapore Airlines’ 49 percent stake in Virgin Atlantic for $360 million, and has agreed to a trans-Atlantic joint venture with the British carrier, expanding its access to capacity-constrained London Heathrow Airport, Reuters reported. Under the joint venture announced Tuesday, Delta and Virgin would share costs and revenues on routes between Britain and North America. The pair plan to cooperate on services between New York and London, with a total of nine daily round-trip flights from London Heathrow to John F. Kennedy International Airport in New York and Newark Liberty International Airport in New Jersey.