"THE TEST of something being really good is if it hangs around for a long time," says singer Jackie Cain. "You think of Gershwin, Cole Porter, Rodgers and Hart, Harold Arlen -- their songs still sound good today. You may find a lyric or two that's a little dated, but most of them are still applicable and wonderful."

Cain's talking about the enduring qualities of songs and songwriters, of course. But she might well be referring to the lasting pleasures of Jackie & Roy, the influential, eternally youthful jazz vocal duo, of which she is one-half -- the other, of course, being her husband, singer/pianist/arranger Roy Kral. The duo appears at Cates through Sunday.

Though both of them are in their good-looking 60s, people still tend to call them "you kids." But it's not as if time hasn't touched Jackie & Roy -- it's just that it's tickled them instead. Known for her clarity, intonation and direct, unfussy delivery, Cain has been called "perhaps the ideal female vocalist" -- in fact, none other than Frank Sinatra once said "you could tune a piano by Jackie Cain's voice." With his warm, fine-grained baritone, supple piano and inventive arrangements, Kral knows intimately just how to support Cain in her vocal flights.

Recently, Cain & Kral were at home in Montclair, N. J., where they had just finished rehearsing a set in their living room with their new bassist and drummer.

"Great combo, a really swinging sound," enthuses Kral after hellos are exchanged. Cain picks up on the upstairs phone.

"Now we've got stereo," she says. Harmoniously married for some 43 years, and performing as a duo for most of those, they've always worked in duet. Why not do an interview that way?

The fluctuating fortunes of jazz and cabaret -- and intermissions for parenting and life crises -- have given their career a stop-and-start-over pattern, and taken them from the gambling rooms of Las Vegas, through a spell as TV jingle singers -- chirping the praises of ("start feeling your") Cheerios and ("muncha buncha") Fritos -- to the supper clubs and cabaret rooms of New York and the world.

Now Jackie & Roy find themselves in the fourth decade of their career with energy and chops to spare, but fewer places to play.

"And you know, it's a lot harder to find new songs now, too," Cain says. "Of course, there are people -- Dave Frishberg and Johnny Mandel come to mind -- who are still writing wonderful songs. But currently I haven't really heard anything that I really like.

"I mean, it's not that we're snobs, but it's just that we like what we like," Cain continues. Then she laughs. "Well, I guess we are maybe; we're maybe a little elitist when it comes to that. I can appreciate what other people choose to do, but for us, we like to do good songs."

So for the past few albums, the couple has concentrated on the songbooks of such time-tested songwriters as Stephen Sondheim, Cy Coleman, Alan Jay Lerner; most recently perfecting a 15-song sampling of tunes written by their old friend, the late composer Alec Wilder, who wrote the liner notes for Jackie & Roy's debut album.

"You don't need to know the song or anything whatsoever about music to know that they're telling you life is worth living," wrote Wilder, " . . . {that} the sun will rise tomorrow and the moon tonight; that love makes miracles and that joy and laughter have not nearly fled the land."

So 33 years later, Jackie & Roy return the favor, devoting "An Alec Wilder Collection" (Audiophile) to their friend "Mr. Wizard's" tunes.

"He always liked people who did good material, and because we fell into that category, naturally it was easy for him to like us," Cain says. "We've been trying to do this Alec Wilder project for a number of years, but a lot of record companies said, 'Who's interested in Alec Wilder?' Everything is judged by how much money it can make, so what if it's artful or aesthetically wonderful, you know? But we wanted to do it because we had such affection for the man and his material."

"All the songs on the album grab us one way or another," says Kral. "Each one of the songs in a way refers to something we can relate to one way or another. For instance, 'Remember My Child' is very poignant, having had two daughters," Cain says.

The album cover features a 1973 photo of Jackie & Roy with a beaming Wilder. "That was one of the few pictures of Alec smiling," Kral says. "He was usually kind of dour in a way."

"Although he wasn't really like that when you were with him," Cain puts in. "He smiled and he laughed a lot."

But then, it's hard not to smile when Jackie & Roy are around.

While Kral says both of them still enjoy the "fascinating craft" of recording, they really live for singing live. And their audiences have a palpable affection for the approachable duo -- it's not unusual to see people linger to chat with Jackie & Roy on a first-name basis after the show.

"Yeah, we have many old friends in all the major cities. In fact," Kral jokes, "sometimes it seems they almost would rather talk to us than hear us sing."