She slings hamburgers in a Long Island fast food restaurant. He programs computers at a nearby electronics company. But after hours Fleurette Arsenault and Danny Littel slip out of such mundane lives and into black-sequined leotards. They lace up their roller skates. They tango, waltz, fox-trot-dance.

It is more than a case of Saturday Night Wheeler. Arsenault, 21, and Littel, 27, are the world's best roller-skating couple. At the Pan Am Games in July they won the gold medal in artistic roller skating, the first year the event had been part of the competition. This week they are in West Germany, seeking their third straight world championship. "It's hard to be graceful on rollers," says ice-skating queen Jo Jo Starbuck, who learned firsthand as a commentator at a Colorado sport festival this summer. "But Dan and Fleurette are so smooth and exciting you don't even notice their skates."

Artistic skating (roller hockey and speed skating are other variations of the sport) is only now attracting attention, partly because of the roller disco craze. Yet Littel and Arsenault have been performing together since 1975. Littel's then partner turned pro and he phoned Arsenault, whom he had seen at previous competitions. The next year they placed second in the world at Rome.

"Before that they didn't exhibit much flair, but once they became champs, they skated like champs," says their coach, Jack Burton. Starbuck adds magnanimously, "Roller skating is much more difficult than ice skating. Only two or three inches of our blade hits the ice at one time; to turn takes only the slightest movement. With rollers, turns take tremendous control."

Arsenault and Littel practice 25 hours a week. In competition couples skate four compulsory dances (waltz, fox-trot, march and tango) and one freestyle. Says Littel, "Free dance has a lot of strange moves and many of our steps come up purely by accident."

Arsenault, daughter of a Boston factory worker, began skating at age 7 and never lacked for rivals. Sister Denise, 25, is now a skating teacher; Claudette, 16, placed second in a junior dance event at the 1979 Nationals; brother Remond, 23, played roller hockey, and David, 18, has danced competitively.

After high school Arsenault commuted to New York weekends to work with Littel and coach Burton for three and a half years. Then she decided to move. "My parents didn't like their little girl taking off for the big city," she smiles. "But if I was serious I had no other choice."

Littel grew up in San Diego and attended Mesa Junior College, intending to be a lab technician. At 5'10", 145 pounds, he swam and played baseball. But he was always a street skater, even if he never placed better than second in serious competition before he partnered with Fleurette.

That partnering "is strictly platonic," stresses Fleurette (for the benefit of a new West Coast boyfriend), though she and Littel share a two-bedroom Farmingdale apartment. "It helps with expenses," Littel says.

The skaters occasionally escape formal training at a New York roller disco called Wednesday's. Says Littel, "Disco is a lot less work than dance skating, but at least it has made people forget the Roller Derby image." Fleurette, however, huffs, "Disco does cheapen the artistic side of skating."

But if art is long, so is the list of expenses for international competition. After an exhibition tour this fall, Fleurette and Danny will turn pro and teach in Orlando, Fla. They are also working on a book, Dancing on Wheels. As one indication of the competitive pressure they are under, both are heavy smokers.

"I forget what a normal life is," confesses Arsenault. "At 7, I had to choose between violin lessons and skating. There's never any time for friends. The older you get, the more difficult the choices get. Many times I've wanted to give it all up." But, insists Littel, "The winning and the glory in the end make it worthwhile."