Andy Gibb, who was mentioned in Sunday's Show section story about "The Pirates of Penzance," was not a member of the Bee Gees although he is the youngest of the brothers Gibb.

CAROLINE Peyton, who demonstrates a sweet and versatile soprano as the ingenue Mabel in "The Pirates of Penzance" at the National Theatre, was once in a rock group called the Screaming Gypsy Bandits. Peter Noone, who plays opposite her as Frederic, was the lead singer in "Herman's Hermits," known for making little girls scream over songs like "Mrs. Brown You've Got a Lovely Daughter."

In the roster of pop stars who have trilled their way through "Pirates" as the demure Mabel, Linda Ronstadt was the first, taking a break from her image as a lusty and soulful rock singer. Maureen McGovern, who now plays Mabel on Broadway, has been a pop singer for 10 years, known for the hit "The Morning After." Andy Gibb, who portrayed Frederic in Los Angeles, was one of the BeeGees. Patrick Cassidy is Frederic in New York--he's only 20, so he hasn't done much at all, but his lineage is perfect for this production: parents who came out of musical comedy (Shirley Jones and Jack Cassidy), and older brothers in rock and pop (David and Sean).

These performers, whose backgrounds are primarily in rock or pop music, are essential components of Joseph Papp's "melting pot" production, in which the concept of musical comedy was substituted for operetta. The collision of contemporary style with Victorian music has worked remarkably well in rejuvenating the venerable Gilbert & Sullivan. Papp was elated with the result.

"We had artists of all different backgrounds," he said, "and it made for a kind of melting pot. I could tell right away that it was going to be good. We had a run-through after the fifth day of rehearsal, and the feelings were so high, they criss-crossed in the most extraordinary way."

And the show allowed the singers to display facets of their talent--like creating a character while leaping, dancing and singing--that had not been seen before, and thus provided a career boost.

"In the recording industry you're only as good as your last record," said Maureen McGovern. "I made a conscious decision two or three years ago not to be dependent on that hit, and began to look for things that would broaden me."

"A rock person brings a lot of guts," said musical adapter William Elliott. "A lot of actor-singer-dancers are trained in all areas but are not great in any single one. We wanted people with charisma and personality, who sing well but not traditionally."

By throwing together the rock and pop singers and more traditionally trained "personalities" like Treat Williams, James Belushi, Kevin Kline and Estelle Parsons in the lead roles, Papp, Elliott and director Wilford Leach--who did the casting--came up with a bonanza not only for themselves but for some of the performers as well.

For Ronstadt, the role provided a part in the movie--her first, recently completed filming in London--and a challenge that was generally applauded by critics. Noone and Karla DeVito, another Mabel, both produced records while they were in the show. McGovern, who got the part right after playing her first stage role as Maria Von Trapp in "The Sound of Music" in Pittsburgh, has performed a nightclub act during the run of "Pirates," gotten other offers for shows (she won't give specifics), and has ideas for two one-woman shows.

"And Maureen doesn't know it yet, but a representative of the Rodgers and Hammerstein estate came to see her and wants to produce a major musical geared to her voice," said press representative Richard Kornberg.

McGovern said she was "terrified" when asked to audition for "Pirates." But she added that the show has strengthened her four-octave voice and exposed her "to a whole wealth of new material. In pop music you never get to see anybody else."

Rex Smith, a teeny-bopper singer and fledgling television actor who played Frederic in the first cast on Broadway, is in the movie and may be doing Shakespeare for Papp this summer. Andy Gibb left the road show early to take a part in a television series. Kevin Kline won a Tony award for his performance as the Pirate King, and has a lead in the movie of "Sophie's Choice."

Many other rock and pop performers have applied for the jobs, say people connected with the show, but have not had the vocal or physical strength to make the grade. Once Ronstadt took the leap, and made a hit with it, others began to see what a dose of Broadway could do both personally and professionally.

"Everybody's had their exposure," said Marjorie Livingston, who has been the vocal coach, helping the rock singers make the transition to Gilbert & Sullivan, the stage and eight performances a week. "I've been impressed with how hard these rock people are willing to work. And I don't think a lot of them knew how hard it was going to be."

When Belushi came to audition for her, she said, he was so nervous he smoked eight cigarettes waiting for it to start. "The first thing I said was, 'You're going to have to cut out those three packs a day, and everything else.' And he said, 'I'll do anything you say; I really want this part.' "

Belushi's athletic Pirate King is a superbly controlled, hysterically funny performance that must require the energy of a professional basketball player. "He's made a lot of progress," said Livingston, sounding like a stern den mother. "This tour ends in L.A., and that will be very good exposure for him."

Peter Noone had made a new record but needed something to get him known after moving to the United States almost two years ago. When he heard about "Pirates," he asked for an audition, and although at 34 he was the oldest one there, he got the part. He has his own opinion of why, aside from the obvious "exposure" benefits, rock singers have been attracted to the show.

"It's good music," he said. "And it's fun."