EVERY Saturday night for 13 years, Lawrence Welk introduced them as "da luffly Lennon Sisters." Dianne (Dee Dee), Peggy, Kathy and Janet, Welk's champagne chanteuses, all-American girls, model wives and mothers, staples of the movie magazines.

Peggy Lennon, 41, is alarmingly cheerful and talkative on the phone at 7 a.m., Los Angeles time. "Oh, I've been up for hours and hours," Lennon says. "I have six kids ages 8 to 17 to get ready for school, and I teach Old Testament, Scriptures and social studies at the Catholic boys' high school here."

The quartet, part of a big, musical, Catholic family (eight children), has sung together since childhood. They were "discovered" when Dianne (the eldest) was dating Lawrence Welk Jr. He brought them home to audition for his dad, who was in bed with a cold. They first appeared in 1955 on Welk's Christmas Eve TV broadcast, sang "He" a cappella, and were an instant hit.

"We have so many people come up to us saying 'You were just like my grandchildren' or 'You were like our own children.' Or more often, 'We had to watch you every week,' " Lennon says with a laugh. "But that's what our staying power is. We grew up on TV, when TV was much less sophisticated. We were family, not TV people. When we got married, people sent us wedding presents."

Even though the Lennon Sisters left the Welk program 13 years ago, audiences still remember the group's squeaky-clean image." We would prefer not to have a Lawrence Welk image, of course. We have grown. We're women." Everyone on the show was clean and wholesome, or they were fired. Which is not to say the Lennon Sisters were anything but wholesome.

"It's just that our tastes in music, dancing and choreography have expanded," Lennon says. "On the Welk show you were stifled in creativity. Just before it ended, the kids on the show were still doing the things we were doing years ago."

The Lennon Sisters still run into Welk occasionally and are friendly with many of the Welk regulars. Like Jo Ann Castle, whose ragtime piano was outrageously decked out each week. "Jo Ann lives about two doors down from Janet. She's recording her own country-western music now."

All the sisters are married and live in the Los Angeles area, within 15 minutes of each other. "We're really sisters," Lennon says. "When we're together we talk about which washing detergent works best and that stuff."

"We're not filled with ambition," Lennon says.

"None of us has a lot of money in our savings, even though we've worked for 26 years. The responsibility of being mothers is much more important than stashing away a million bucks. In a sense, we're struggling away just like any other mother in the U.S." Except these suburban moms zip off for a week's gig in Las Vegas or Atlantic City, where they opened Wednesday for four days at the Claridge Hotel and Casino.

Lennon describes the current act as fast-moving and contemporary. "But we don't do hard rock. We do 'Jump Shout Boogie' and 'Last Dance,' and the old songs people expect to hear from us, a cappella things."

And yes, it's true: the Lennon Sisters take off their clothes on stage. Zipped into form-fitting, side-slit sequined gowns (with red corsets underneath, for heaven's sake), the four all-American moms do a "striptease" as part of their "Cabaret" medley. "Of course, it's in very good taste," Lennon says. "People are so surprised that we're not little girls, we don't just stand in ruffles in the corner and sing 'Cruising Down the River.' "