MIAMI — Southwest says it is returning a pair of slots it received from American Airlines at Washington’s Reagan National Airport (DCA) to the FAA. It had won 27 slot pairs, giving it the right to operate 27 daily round-trip flights.

Southwest said it won a Sunday-only pair that it does not intend to use. The company told USA Today that “The Sunday only slots were not something we could use and we did not purchase them, they were included in our bundle…we will be returning them to the FAA. We do operate some Saturday only schedules in various location, but not Sunday only.”

To clear the way for American and US Airways to merge, both companies agreed to give up 52 slot pairs at DCA.

Since this is out of the normal, it is not known who will handle the process of returning the slot pairs.

JetBlue Airways won 12 DCA slot pairs, and Virgin America won four DCA slot pairs last week. All of the slot pairs have been accounted for, but the fate of one remains unknown.

It is quite rare for an airline to voluntarily give up a slot pair, especially at DCA, as they are highly coveted. One slot pairs provide airlines with the right to operate one take off and one landing. They are common in highly congested airports where operations are heavily restricted.