(FORTUNE Magazine) – Whatcanigetchabuddy?

It's 11:30 P.M., and the bartender is waiting for your order. After another 15-hour day at the office, you couldn't care less what you drink, so long as it has alcohol in it. Your eyes skip down to the end of the bar to a couple in their early 20s who remind you of more carefree days. Each is sipping from an oversized martini glass brimming with an iced-tea-colored concoction. "I'll have what they're having," you tell the bartender, with a sign of resignation, wondering how they can afford a $9-a-drink joint like this.

What you didn't know at the time was that those kids were getting paid to sit on their duffs and sip those Hennessy martinis. They were players in a massive stealth marketing (how's that for a contradiction in terms?) campaign that New York-based "communications company" Kirshenbaum Bond & Partners ran for five years on behalf of Cognac Hennessy. At its peak the campaign--which ended last New Year's Eve--was planting live decoys in bars across the U.S. 365 days a year to influence the tastes of 21- to 32-year-olds cynical about traditional advertising.

Here's how it worked: In New York, Chicago, San Francisco, L.A., and Miami, Kirshenbaum Bond recruited attractive young hipsters by working within their network of friends. The recruits were asked to convene in the conference room of a top-drawer hotel if they were interested in making some easy money. At the hotel, they were photographed and asked about their drinking habits, as well as their social life.

Those that made the cut were let in on the secret: they had been hired to help the firm popularize the Hennessy martini and its sweeter and more accessible sibling, the Hennessy sidecar. The idea was to boost sales of Hennessy cognac by showing people that it was no longer something just old fogies drank out of snifters. Everyone was told to hit the swankest bars in town (i.e., San Francisco's Red Room) and order plenty of Hennessy cocktails.

If a bartender hadn't heard of the drinks, the undercover shills were told to express mock surprise--"Really? Everyone in New York is drinking this"--and explain how to make it. They were also encouraged to pick up the tab for their friends or anyone they met at a bar. The only no-no: they couldn't say or do anything that would blow the campaign's cover.

Every week, members of the covert sales force had to fill out detailed forms packed with questions like "Did other people order the drink after you?" For their trouble, they were paid as much as $50 a night and reimbursed for their bar tabs. Presumably the compensation was a little more generous for the celebrities who were also in on the act. Kirshenbaum Bond won't kiss and tell, but if you've seen a picture of a movie star holding a reddish-brown concoction punctuated by a ribbon of lemon peel, chances are that starlet was also on the firm's payroll.

Amazingly, Kirshenbaum Bond succeeded in keeping the campaign a tightly held secret until last December, when Michelle Goldberg, a journalism student at U.C. Berkeley, penned an account of her stint as an undercover liquor promoter for the online magazine Salon. While Goldberg didn't say which company she worked for, the Hennessy campaign ended within weeks of the article's appearance. Kirshenbaum Bond insists that this is merely a coincidence, that the campaign ended because it had successfully created a new market for Hennessy. The firm is also quick to point out that while it invented this supersoft sell, it's been widely imitated and is being used to market at least a dozen different beers, liquors, and cigarettes.

Does stealth marketing really work? Well, the secret Hennessy campaign, combined with a more traditional marketing blitz that splashed ads on everything from phone booths to matchbooks, appears to have paid off big. Sales of Hennessy cognac have been on an upward swing for the past four years and broke the one-million-case barrier in the U.S. for the first time in 1997. With results like that, it's only a matter of time before this newest marketing ploy becomes, like product placements in movies, one of those "secrets" that everyone knows. But the new era of stealth marketing might not be as bad as it sounds. Given all the impure motives someone can have for buying you a drink, the idea that they might just be doing their job ought to be downright reassuring.

--Ed Brown