if a woman is married but kept their last name

do i address her as Ms. or Mrs?

Re: if a woman is married but kept their last name

Ms.

from an etiquette guide....

When sending to a married woman who uses her maiden name at work, address the cards to “Mr. and Mrs. John Smith.” If she prefers to use her maiden name for all occasions, work and social, address the card to “Mr. John Smith and Ms. Susan Jones.”