Southwest Airlines and Calgary, Canada’s WestJet have made an agreement to combine routes, flight schedules, fares and frequent flyer programs by late 2009. Travelers will be able to use Southwest’s Web site to book flights on WestJet as well as Southwest for seamless travel.

WestJet flies to 28 destinations throughout Canada, 12 destinations in the U.S. including Hawaii and nine destinations in Mexico and the Caribbean, including the Bahamas and Jamaica.

Southwest has not had an airline partner since its lone partner, ATA Airlines, went out of business earlier this year. “We are quickly moving forward with our plans to enter the international market,” Southwest’s CEO Gary Kelly said. WestJet is banking that this partnership will draw more business travelers to the carrier. Codeshare details and other elements of the partnership are subject to approvals by both the U.S. and Canadian governments.

CNNMoney.com reports that Southwest Airlines plans to expand seat capacity by four percent this year, eliminating 31 flights, but with plans to add 40. WestJet expects to expand capacity by 16 percent this year, when most airlines are reducing capacity.