They just couldn't believe it. Cher! In Franconia!

But there she was last night at the Wheel-A-While roller disco in her purple leopard skates and black sparkly pants, skating on a Monday night, just as she does on Mondays in Hollywood. Some 500 adolescents gawked in utter awe.

It was absolutely the biggest thing to hit this Alexandria suburb since, well, since Fort Hunt High School burned down last December. And to Pam Gore, a 14-year-old who has spent most of her summer in front of the television set, it was pretty much the hottest thing in her life so far. Unless Leif Garrett, blond heart-throb of Tigerbeat magazine, asks her out. But that hasn't happened yet.

"I just can't believe it," said Gore, which is just about what everybody else said, too.

The crowd had lined up outside the Wheel-A-While on Franconia Road since 5:15 p.m. By 7:30 they were skating, anxiously counting the minutes until Cher was due to arrive at 11, straight from her concert at the Kennedy Center.

She arrived late, in true rock-star fashion, about 11:45, and had a lot of second thoughts in the roller disco office before she made her appearance.

"I'm going to get killed in that crowd," she said.

But after huddling with her manager and Scott Shannon, the program director for WPGC - the top-40 radio station that arranged the $4-per-person roller benefit for UNICEF - Cher decided to brave it.

And they loved it. True, Cher didn't sling her famous hair and shake her skinny legs, but she moved slowly and carefully through a crowd five times the normal size, smiling as she went.

She came back after one lap around the ring. "It wasn't going to work," she said, fluttering her purple lids as she took off skates to reveal sparkling maroon toenails. "Someone was going to get hurt."

But that one turn around the ring was all the crowd needed. And for Marc Toms, a 22-year-old computer operater from Olney, Md., it was ecstasy. Toms has been a Cher fan since he was 9 years old and has a 3-by-4-foot collage of her in his room.

But for Toms, Cher's appearance last night wasn't quite as ecstasy-filled as it was five years ago. That was in Gaithersbury, outside the now-closed Shady Grove Inn, where he waited eight hours in line for her autograph. The doors closed when he was only four people away from the goddess.

"I sat down and cried. But then, 20 minutes later, the door opened and she came out and gave me her autograph and kissed me on the cheek. It was worth everything." CAPTION: Picture, Cher at the Wheel-A-While in Franconia, by Joe Heiberger