Beam me up, Scottie. I'll pay by Trekkie card

Karen Giddings enjoys talking to strangers. Which is just as well, because every time she uses her credit card to pay a bill Karen ends up striking up a conversation with the person who accepts it.

Her card is issued on behalf of the Star Trek fan club. It is emblazoned with a scene from the famous TV and big-screen series, forcing her to explain where it came from whenever she hands it over.

More than 12,000 "Trekkies" and science fiction fans use this card for their weekly shopping, or anything else for that matter, marking them out as one of hundreds of "affinity groups" each with a distinctively designed piece of plastic strictly for them.

Hundreds of thousands of card users are doing nicely with credit cards issued by organisations as diverse as the National Trust and the RSPCA, the Campaign for Real Ale and the National Association of Fish Fryers.

Karen, a marketing executive, is one of them: "I got mine about two years ago. When I was a student, we used to watch every episode by satellite in the house I shared.

"When I heard there was a card I applied for it immediately. I tend to use it a lot of the time when I am travelling or to buy clothes. It does generate a lot of comment. People to want to know how I got it and where they can get one.

"Some people are also fans. They seem disappointed that they can't get pictures from different series and different people."

The market for affinity credit cards like this has exploded in the last five years, with some 2 million users capturing about 7 per cent of the market.

In each case some kind of donation is made to the affinity organisation. In the case of the Bank of Scotland, one of the largest players in the market, this is a contribution of 0.25 per cent of the spend on all cards, plus an "activation fee" of around pounds 7.50.

Bank of Scotland employs TransNational, an American company with its UK arm based in Reading, to seek out suitable affinity groups. Since the late 1980s, when the bank began issuing, it has donated almost pounds 9m to more than 420 causes and affinities.

Bank of Scotland is rivalled in the market only by US firms Beneficial Bank and MBNA, although Halifax has a number of affinities and Co-op Bank has some big member cards - the Labour Party and Amnesty International among them. Even Midland has a few.

The rewards for many of the groups are often too attractive to miss out on. The RSPCA, which has more than 70,000 in circulation, has raised slightly less than pounds 1m since it was launched in 1993. A medium-sized charity group, Action Aid, for example, has benefited by around pounds 400,000.

The Star Trek card attracts users who are science fans, as well as genuine "Trekkies". It is marketed through advertisements in specialist magazines and cable TV science fiction channels. TransNational is planning a major TV advertising campaign to promote its benefits, beginning next week

David Williams-Jones, TransNational's divisional general manager says: "Consumers are polarising in their views and, whether they are doing it for themselves or because they want to do a little bit for others, I don't know. But there are massive numbers of people coming into the market.

"The advantage to these groups is that money comes in on an annuity basis, so each charity knows that a certain time of year it will receive a certain sum."

This, perhaps, is why around half of all the affinity cards on the markets back charities. Another 20 per cent are held by groups such as the Trekkies, 15 per cent by alumni organisations like the University of Manchester, 10 per cent by sporting clubs like Coventry City Football Club and the remainder by a variety of clubs and societies such as Mensa.

Any lawful organisation with a database of more than about 7,500 members can find a deal to issue its own cards, although most issuers would baulk at politically or socially sensitive affinity groups.

Professor Steve Worthington, professor of marketing of financial services at Staffordshire University Business School, has made a study of affinity cards.

"There has been tremendous growth in this market. But [it] is, by and large, for those who missed the boat originally," he says.

Part of these cards' popularity is definitely to do with the shared activities of those who take them out, he adds.

Whether it is just novelty value or a true sense of altruism which has caused this explosion in the market remains something of a mystery.

In general, affinity cards are not even necessarily good value. APRs and fee structures vary as much as the conventional credit card market. Bank of Scotland, for example, offers a standard 20.9 per cent APR rate on all its cards. That can be bettered by many competitors.

Among them are Save & Prosper, which charges just 12.4 per cent APR on purchases, Saga, which charges 16.9 per cent, and Co-op's Advantage card, which costs 10.9 per cent APR but has no credit limit.

Not that this bothers Karen Giddings: "To be honest, the rate doesn't bother me. I have arranged my finances so that I pay off my credit card bills each month. The important thing is that I get credit every month - with a card I enjoy using."

Star Trek card: 0845 6044460.