Hap Hansen of Encinitis, Calif., rode Mirage and Alice Debany of North Salem, N.Y., rode Tarco through quick jump-order changes for the fastest finish to win the Costume Pair Relay at the Washington International Horse Show last night at Capital Centre.

In the international open jumper event, the teams could alternate which horse jumped each fence, as long as the team completed the course twice, and took each jump in proper sequence. With the final result decided by fastest time, this meant a lot of strategizing as the teams tried to cut corners and save time. Faults, or fence knockdowns, were converted into seconds.

The winners made many jumping changes, on the advice of U.S. Coach Frank Chapot, alternating after maneuvering one or two fences each.

The U.S. Equestrian Team members finished the course cleanly in 58.43 seconds to beat the field of 11 other teams. USET members Tony Font of Houston and David Raposa of Clinton, N.Y., were fourth on Benny and TNT respectively.

Donald Cheska of Waukesha, Wis., scored his second runner-up finish in as many nights with his mount Fury Too and partner Mark Leone of Franklin Lakes, N.J., riding Crown Royal Albonnie (60.98).

Chris Kappler of St. Charles, Ill., rode Warrant and Anne Kursinski of Flemington, N.J., rode Cannonball for the third-best round (61.28).

Four teams were disqualified for jumping out of sequence as they attempted to cut their times.

With the team competition continuing each night through Sunday's finale, the U.S. team leads with 16 points, followed by Canada with six. Britain and France have not yet gained a point. Debany's share of the victory tied her with Cheska atop the rider standings with 10 points each. Hansen has seven points for third.

In earlier events Meant To Be, a leader in hunter classes this week, was first in the regular conformation, under Scott Hofstetter.

Tindle, owned by Bertram Firestone of Waterford, Va., and ridden by Katie Monahan Prudent of Middleburg, Va., won a blue ribbon in another regular conformation hunter class and fourth in the model division. Bib and Tucker was ridden by George Lindemann to finish second in both conformation classes. Gene Kelly, was ridden by Tommy Serio, to third in both divisions for owner Catherine B. Peterson of Cockeysville, Md. RESULTS

International Costume Pair Relay: (rider, horse, rider, horse, time) Hap Hansen, Mirage and Alice Debany, Tarco, 58.43 seconds; Donald Cheska, Fury Too and Mark Leone, Crown Royal Albonnie, 60.98; Chris Kappler, Warrant and Anne Kursinski, Cannonball, 61.28; Tony Font, Benny and David Raposa, TNT, 63.79; Hugh Graham, Mookie and Beth Underhill, Amstel, 64.79; Francie Steinwedell, Quivent and Candice Schlom, Sold Out, 72.59.