UPDATE 1-Southwest plans Mexico codeshare

NEW YORK, Nov 10 (Reuters) - Southwest Airlines Co (LUV.N), the U.S. No. 1 domestic carrier, is to start a codeshare partnership with Mexican airline Volaris, extending the U.S. airline's plan to access international markets.

Southwest, which is in the process of forging a similar partnership with Canada's WestJet Airlines (WJA.TO), said flight schedules and details of the codeshare would be hammered out by early 2010.

Under a codeshare, airlines can sell tickets on each others' planes, reaching new customers while saving money on marketing.

Dallas-based Southwest, which invented the low-cost airline model 37 years ago, serves only U.S. destinations, but is looking to expand its network even as rivals cut capacity in the face of volatile oil prices.

Volaris, founded in 2006, flies single-aisle Airbus (EAD.PA) jets between 23 cities in Mexico. (Reporting by Bill Rigby, editing by Matthew Lewis)