Excuse me, but aren’t you ... ?

Group cycling instructor Darryl Gaines never expected to see his idol Diana Ross pop into his class at the Sports Club/LA, but there she was. Not only did she pedal her way through the entire class, but she reveled in exercising to some of her hits, then stayed for a bit to bask in the adulation of her fellow cyclists.

As Gaines tells it, “It was almost like a dream. It was a dream. She was so nice to everyone. They all went up to her afterward and told her how they had gone on their first date and heard ‘Baby Love.’ It was amazing. Amazing.”

Don’t think for a moment, however, that this is typical of celebrity encounters at gyms. Ross obviously set the tone for the visit, which happened a few months ago. Most stars don’t invite that kind of attention, instead preferring to keep to themselves, or going so far as to hide behind hats and sunglasses.

Any savvy Angeleno will tell you that the etiquette of celebrity-spotting in general usually requires a discreet reaction. No fawning, no gushing -- just a silent acknowledgment (and maybe an elbow nudge to your friend) that, yes, you are in the presence of somebody really, really famous. It’s no different at the gym.

But star sightings at health clubs are inevitable in this company town, and those who work out in the L.A. area have tons of stories to tell, such as spotting rocker Dave Navarro pumping iron solo at Crunch (sans wife Carmen Electra) or seeing Jessica Simpson doing cardio at the Sports Club/LA Beverly Hills.

It’s a unique situation, because celebrities are often makeup-less, hairdo-less and stylist-less, with no obsequious flack around to fetch them a Fiji. You and they are almost like peers. So there’d be no problem taking the next elliptical over and starting a schmooze-fest, right?

Uh-uh. Approaching a star at the gym just for the sake of approaching a star at the gym isn’t done. Just because you’re both members doesn’t entitle you to a private audience to talk about your latest script or comment on their latest romantic escapade.

Don’t just take my word for it. The official scoop from etiquette expert Jeanne Hamilton is this: “I believe they should be treated no differently from other people when it comes to basic courtesy,” she says. In other words, if you regularly nod and smile and say “Good morning” to your sweat-mates, you can do it to famous people as well. Do not, advises Hamilton, interrupt their conversation, stare, or strike up an intimate chat.

“We think we’ve known them our whole lives because they’re on the air or in movies,” says Hamilton, creator of the popular “Etiquette Hell” website (billed as “the Internet’s largest repository of bad etiquette”) and author of two books on wedding etiquette. But because they’re not our best friends, she suggests starting with something along the lines of “Hi,” which tosses the proverbial ball into their court. Note to you: If no ball comes back, don’t pursue it.

Yes, there is something of a level playing field in a gym. But, as Hamilton points out, it pretty much ends there. You probably won’t be spending the first weekend of March 2006 at the Academy Awards.

Not that the urge to do or say something doesn’t strike, even to some who are considered peers. Bruce Eric Kaplan, television writer (“Six Feet Under”), producer, author and New Yorker cartoonist, recalls seeing actress Marcia Cross in his yoga class when her show “Desperate Housewives” was beginning to take off.

Kaplan says he briefly felt “that the whole class should give her a standing ovation to say ‘That’s right, you’re on top of the world, you’re back and you’re better than ever. You’re a survivor. Yea, Marcia!’ ” Of course, he restrained himself. “It’s probably the last thing she would want,” he says. “She appears to be a very private person.”

As do most celebs when they work out, be it at an open-to-every-plebeian-type gym or one of those small, exclusive private facilities where members come with their uber-trainers. In recent years gyms have become prime stakeout venues for paparazzi hoping to catch stars au naturel, which makes one wonder why a celeb wouldn’t just work out at home.

At a club, says Mylene Dane, general manager of the private gym Train in West Hollywood, “there’s more equipment, and they get more motivated. I’ve had so many trainers who say their clients want them to come to someone’s home, and all they have is a cardio machine. They get kind of bored.”

Dane says she never advertises that celebs work out there. But having stars as clients certainly doesn’t hurt a facility’s reputation. That’s why Yoga Works put out a recent news release that described the chain of studios as one “that’s been drawing A-list celebrities including Gwyneth Paltrow, Kate Hudson, John Cusack and Diane Lane for the past 18 years.”

The company did that, says public relations director Amy Morris, “because celebrities have everything in the world available to them, access to the best trainers, and they choose to come to the studio. That says a lot about the studio.”

If it’s a major faux pas to make a big deal about seeing stars at the gym, Hamilton says, it’s perfectly fine to call your friends afterward and tell them.

Even Dane acknowledges to an occasional private freak-out over seeing a celeb at her own gym: “I do that in my office,” she says. “I’ve lived in L.A. all my life, and I still get so star-struck. But outside, I’m very cool.”

Gaines, at Sports Club/LA, realized he could dispense with coolness during Ross’ recent visit. Not only did he play a number of her songs during class -- which, he says, she loved -- he even fit her foot into the pedal.

“For a moment,” Gaines says, “when I was putting her on the bike and adjusting her seat and putting her foot into the pedal, I thought, ‘I have Diana Ross’ feet.’ ”