When you wish upon a star... : MEDIA

Using a celebrity to plug a product is the oldest advertising trick in the book. Ronald Reagan, ex-American president, has been doing it for years - promoting GE Electric in the 1990s with the same gusto he applied to Chesterfield cigarettes in th e 1950s. But the industry remains divided on its merit.

To many, a celebrity endorsement is a cheap trick to hide a weak creative idea. To others, the publicity-seeking promiscuity of some "names" - the Tony Slatterys, the Angus Deaytons - automatically devalues their usefulness in a campaign. To many agency creatives, however, the chance of linking a star name to a particular product is irresistible.

Cost can prove a disincentive. Dudley Moore reportedly received £500,000 for his three-year contract to chase chickens around the world for Tesco, although fees usually start at £50,000. Then there is the danger that the celebrity might rubbish the product. Griff Rhys Jones, for one, made fun of Holsten Pils after he was dropped from its campaign. Similar games have been played by Jerry Hall (ex-Bovril) and Paul Gascoigne (ex-Brut).

Some celebrities can be decidedly snooty. One answer is to encourage their involvement in campaigns outside their home countries. Japanese viewers have enjoyed Arnold Schwarzenegger selling noodles, Madonna plugging TVs and Bruce Willis endorsing mobile phones - activities which might cause some to shrink in embarrassment if ever shown at home.

The up-side is the PR potential. When a campaign has run its course, such as the "Man in Black" campaign for Guinness featuring the Dutch actor Rutger Hauer, fevered speculation dominates the press. The burning question is: "Who next?" (Guinness fans will find out in March.)

But even if an ad agency does get it right, success is never guaranteed. Advertising folklore has it that as a result of the Cinzano campaign in the 1970s - featuring Leonard Rossiter and Joan Collins - sales of Martini soared. Earlier this month, despite the best efforts of Harry Enfield, Mercury admitted defeat in part of the consumer telecoms market.

However, Mr Enfield - unofficially dubbed "the voice-over King" - can take some consolation from the fact that his voice is heard on more commercials than any other 1990s name.