New York (AFP) – Budget US carriers Spirit Airlines and Frontier Airlines announced Monday they will merge to create a competitive low-cost airline that aims to test the dominance of larger rivals. The cash and stock deal is valued at $6.6 billion and expected to close in the second half of the year, pending approval from US antitrust regulators. Executives said the deal will challenge the “Big Four” US carriers American, United, Delta and Southwest by expanding low-fare options for service in the United States, the Caribbean and Latin America. “This transaction is centered around creating an …