Several U.S. airlines have introduced a new twist into their highly competitive frequent-flyer programs, enabling travelers to earn bonus points faster than previously for free travel. The airlines have joined with Visa or MasterCard (or both) to issue special multipurpose charge cards. Every time you use one, you are credited with bonus mileage points at the rate of one mile per dollar spent.

Among the airlines featuring such cards are American, Eastern, Continental, Piedmont and United. American calls its card the AAdvantage Card; Eastern and Continental are linked with a Gold MasterCard; United is issuing the Mileage Plus First Card. The Piedmont card is simply the Piedmont card.

Meanwhile, Eastern and Continental have added yet another new twist by bringing out the OnePass, their revised frequent-flyer program. Breaking from customary procedures, the two airlines have reduced the number of miles needed to qualify for free travel -- if you fly during off-peak times. They also will allow you to buy bonus points if you are short of the number required for an award.

The two airlines tout the OnePass as "absolutely the fastest way to earn free travel."

Both of these new industry marketing devices -- the charge card that earns mileage and the OnePass -- add a new dimension to the highly popular frequent-flyer programs. At this point, it is too soon to tell how many other airlines will duplicate them -- or come up with yet another innovation.

When United announced its new charge card May 26, then-Allegis Chairman Richard J. Ferris cited this example of how it works:

Business travelers who fly 50,000 miles a year, he said, "on average will use plastic for purchases during the same year amounting to about $25,000. In other words ... customers traveling and spending at that level would earn 50,000 miles for their travel plus 25,000 miles for their spending," making the total mileage credit 75,000 miles.

(Despite the recent upheavel at Allegis, a spokesman said the card would continue to be offered as advertised.)

On a typical trip, Ferris said, travelers earn miles when they fly, rent a car, stay at a hotel, go out to dinner and buy gifts for the family or items for themselves and their home -- provided, of course, that all of this is charged on the airline charge card. The cards are good at the 5 million establishments worldwide where Visa and MasterCard say they are accepted.

The dollar-a-mile rate is standard, but otherwise each airline's card varies. Most award you extra bonus miles for signing up, but United hands out a $25 credit for use on future travel. The annual fees to obtain a card range from $15 for Piedmont to $50 for American. Interest rates on charges differ by one or two percentage points.

Most of these cards permit you to obtain cash advances from automatic teller machines, but the maximum amount differs. Each airline also offers its own package of incidental travel insurance, including insurance for travel accidents and baggage loss.

Additional cards can be obtained for family members at no extra cost but all miles earned will be credited to the primary card holder's account.

Obviously, travelers considering such a card should pick the one issued by the airline on which they fly most frequently and are building mileage points toward an award. Applications for the cards can be obtained from individual airlines.

Among the charge cards available:

American: With Citibank, American issues two AAdvantage cards, a MasterCard and a Visa card. The annual fee is $50, but there is no charge for the first six months. Applicants receive 2,500 bonus miles in American's frequent-flyer program when they sign up, and another 2,500 bonus miles when they make their first purchase with the card. The annual interest rate on purchases is 17.8 percent, and on cash advances it is 19.8 percent.

Eastern/Continental: The two airlines, both a part of the Texas Air Corp., offer a Gold MasterCard through Marine Midland Bank. The annual fee is $26 a year with no charge for the first six months. When you sign up, you get 2,500 bonus miles and another 2,500 after the first purchase. The initial credit limit is $5,000. The annual interest rate is 18.95 percent.

Piedmont: Piedmont's is a Visa card from the North Carolina National Bank. Actually, it's two Visa cards. The "classic" has an annual fee of $15 and a $1,000 initial credit line. The "premier" costs $25 a year with a $5,000 credit line. There is no charge for the first six months. You get 2,500 bonus miles to sign up and another 2,500 when you first use the card. The annual interest rate is 17.94 percent.

United: United's Mileage Plus First Card is a Visa card issued through First Chicago Corp. The annual fee is $45, but there is no charge for the first year. When you sign up, you get a $25 travel certificate for use on United or with other Allegis-linked companies -- Westin and Hilton International hotels and Hertz car rentals. When you first use the card, you get a package of upgrade certificates for each of the companies -- coach to business/first class on United or standard to deluxe room at Westin, for example. The credit line ranges from $2,000 to $50,000. Through August, the interest rate is 16.9 percent. Afterward, it is the prime rate plus 9.4 percent.

In unveiling the OnePass, the second new development in frequent-flyer programs, Eastern and Continental managed to take a deft poke at United, American and some other airlines. Earlier this year, these airlines increased the number of miles required for some awards, but in the face of protests from passengers they later rescinded them.

Now Eastern and Continental are promising a "redemption guarantee." If in the future the two air partners move to increase mileage requirements, they will give a two-month warning notice to OnePass participants. In that period, you can choose which award you are seeking, and you have up to three years to earn it at the lower mileage level.

"While we always keep changes to a minimum," says marketing vice president Mike Ribero, "it could become necessary to alter reward levels at some time in the future. Our redemption guarantee assures {that} our customers won't see their dream vacation slip through their fingers just as they reach out to take it."

The OnePass combines what was formerly the Eastern Frequent Traveler Bonus Program and Continental's TravelBank. Among its other new highlights: Lower reward levels: OnePass participants who elect to travel at off-peak times (usually noon Monday through noon Thursday) can earn free tickets for fewer miles. For example, it takes 35,000 bonus miles to earn a free round-trip coach-class ticket in the mainland United States and Canada during peak hours. In the off-peak period -- this off-peak program is dubbed "MileageSaver" -- the requirement is only 20,000 miles. There are some additional restrictions. Travel must include a Saturday-night stay and MileageSavers are not available from Dec. 15 to Jan. 5. Mileage purchase: This is an option that, so far, is unique to Eastern and Continental. You can buy up to 15 percent of the mileage needed for an award above the 35,000-mile level. The cost is $10 per thousand miles.

As Eastern explains: "A OnePass member with 75,000 miles, who wants to use a total of 80,000 miles for a free trip for two to London or South America, will be able to purchase the additional 5,000 miles for $50." The option is not available for MileageSaver awards. The top award: "The ultimate reward," says Eastern, is a trip for two every week for a year anywhere Eastern and Continental flies. Called the MillionAir, it goes to the traveler who flies a million miles on the two airlines. Included are 52 round-trip flights, 52 nights in a deluxe room in selected hotels and 52 days' use of a luxury rental car.

"DANCING ON THE WAVES": This is the appropriate name of a new ballet to be premiered next May aboard a Caribbean cruise ship by the Miami City Ballet and its artistic director Edward Villella. Villella, a former star of the New York City Ballet, has led the company to growing prominence in recent years.

The unusual 10-day cruise, a benefit (in part) for the Miami City Ballet, will feature on-board performances, films, lectures, rehearsals and master classes. Passengers can attend or even participate.

The new work, "Dancing on the Waves," was commissioned by South Florida Cruises, a travel firm that offers cruises, including this one, at discount prices. It is holding 430 cabins at reduced rates for passengers making early reservations, according to spokeswoman Madge Mills. A percentage of the proceeds from these cabins is earmarked for the ballet company.

The ship is the Sitmar Fairsky. Departure is May 15 for Nassau in the Bahamas, St. Croix and St. Thomas in the U.S. Virgin Islands and Antigua, Barbados and Martinique. Cabin prices range from $1,970 to $3,540 per person.

However, South Florida Cruises is offering an $800 discount per cabin ($400 per person) for reservations received before Aug. 31. From Aug. 31 to Nov. 30, the discount is $650 per cabin. From Dec. 1 to Feb. 15, it's $500 per cabin. Afterwards, the discount is $300 per cabin. Discounts are not applicable for suites and cabins in the higher-priced A and B categories.

For information: South Florida Cruises, 2005 Cypress Creek Rd., Suite 207, Fort Lauderdale, Fla. 33309, (800) 327-SHIP.