Incredible as it may seem, ABC reports that in the 12 major Nielsen cities Sunday between 11 a.m. and 4:45 p.m. -- during which the Indianapolis 500 was not being run in the rain -- coverage averaged a 6.6 rating and a 22 percent audience share . . .

If that average holds up nationally, some 5.6 million TV homes will have tuned in to watch the rain fall on the Brickyard, catch a 20-minute glimpse of the Hands Across America celebration or stare at reruns of 500s past . . .

In Washington, during those same 5.75 hours, the raindrops on Channel 7 attracted a 5.8 rating and a 20 share . . .

On Sunday evening, "Hello Again," the Neil Diamond special on CBS, averaged a 14.4 rating and a 29 percent audience share in Nielsen's 12 big markets between 8 and 9 . . .

From 9 to 11 that night, "Perry Mason: The Case of the Notorious Nun" on NBC averaged a 21.3/38, leaving Part I of "The Right Stuff" on ABC with a 10.8/20 . . .

Monday night, the Bob Hope special on NBC from 8 to 10 p.m. averaged a 17.2/35 in the 12 major markets while NBC's "You Are the Jury" at 10 p.m. did a 15.4/25 . . .

The 9-to-11 conclusion of "The Right Stuff" on ABC averaged an 11.8/19 . . . while the regular CBS lineup from 8 to 11 did an 11.7/19 . . .

We thought a coatless Dan Rather looked smashing in his yellow sweater vest Monday night on the "CBS Evening News," but he said yesterday the Memorial Day attire does not portend "a trend" . . .

"It seemed the natural thing to do," he explained. "I was in, working on a holiday -- that's the way it is in the news business. I was feeling good and more relaxed. I didn't put a coat on for promos and as time for the broadcast drew near I felt comfortable leaving the coat off" . . .

Rather has previously flouted the strict Anchorperson Dress Code with wintertime sweaters worn under a coat. Whether this latest move leaves The Code in, well, disarray, remains to be seen. But somehow it worked for CBS Monday night on a broadcast that caught the country nicely -- on the holiday road, at solemn rites of remembrance and at organized play, doing good . . .

Over at "NBC Nightly News With Tom Brokaw" -- which marked Memorial Day by taking a whack at George Bush -- Brokaw, quite correctly, kept his coat on . . . as did Richard Threlkeld on "ABC World News Tonight Without Peter Jennings" . . . Also in the News

WETA holds its annual Business and Industry Breakfast at the Mayflower Hotel on Thursday, June 5 . . .

Designed as a kind of "thank-you" by WETA to the more than 50 area corporations that help keep Channel 26 and FM 91 on the air, the breakfast is also open to members of the public -- who can also receive thanks at a tax-deductible $150 per person . . .

Satirist Mark Russell will be the guest speaker. The event, presented in conjunction with Sovran Bank/D.C. National, starts with a 7:45 a.m. reception in the Chinese Room, followed by breakfast at 8:20 . . .

For more information, call 998-2635. That's 998-2635 . . .

CBS has added a fourth entry to its late-night schedule for next fall . . . a new action series about a government security agent called "V.H. Adderly" . . .

"V.H." joins holdover series "T.J. Hooker" and "Nightheat" plus another new action series, "Crimeworld" . . .

Fans of the Australian series "Return to Eden," which concluded a 21-week run on Channel 20 the night of May 10, better not count on catching up with missed episodes . . .

The series performed poorly here and at other Taft Broadcasting stations and, although WDCA has rights for another run, chances are poor for a repeat . . .

Cathy Lasiewicz, senior producer of the weekend editions of "CBS Evening News" since last July, has been named national assignment editor for CBS News . . .

She succeeds David Fitzpatrick, who has been named a producer of the investigative unit of "CBS Evening News With Dan Rather." Lasiewicz joined CBS in 1982 after four years with WPLG in Miami . . .

And general assignment correspondent Marlene Sanders has signed a new five-year contract with CBS. She anchors several editions of "Newsbreak" each week, reports for "Sunday Morning" and the weekend "Evening News" and does considerable anchoring on CBS Radio . . .

Tony Danza, star of ABC's "Who's the Boss?," will star in an NBC movie called "Doing Life," which is based on the true story of Jerry Rosenberg, a convict serving a life sentence who finds "personal salvation" by becoming the nation's first jail-house lawyer . . . Moving Right Along

Here is a list of the mini-series in the works for NBC during the 1986-87 season . . . starting with the 4-hour "Sidney Sheldon's Rage of Angels: The Story Continues," an early favorite for both the snappiest and longest-title titles of 1986-87 . . .

In "Continues," Jaclyn Smith reprises her role as Jennifer Parker, the "high-powered Manhattan trial lawyer whose personal and professional ambitions are jeopardized by an ongoing romantic liaison with a dashing political figure (played by Ken Howard, who, the last time we looked, was getting a little plump for anything beyond a very short dash) and an inexorable tie to a dark underworld figure (Michael Nouri)." . . .

"The Two Mrs. Grenvilles" (4 hours) stars Ann-Margret as "a working-class woman who marries into an aristocratic family and becomes involved in the murder of her husband, the only heir to the vast Grenville family fortune." With Claudette Colbert and Stephen Collins . . .

"Fortunate Pilgrim" (4) stars Sophia Loren as "a strong-willed Italian immigrant who keeps her family together despite great obstacles and struggles to achieve the American dream." Adapted from Mario Puzo's autobiographical novel . . .

"Jackie Collins' Lucky" (7) traces the "colorful lives of Gino Santangelo, a hungry street kid who becomes one of America's most wealthy and powerful gangsters, and his sensual daughter Lucky." Based on Collins' Prix Goncourt-winning novels, "Lucky" and "Chances" . . .

"Nutcracker" (6) is an adaptation of Shana Alexander's reportage of a "true-life case that involves murder and madness and a young man so obsessed with pleasing his mother that he will kill his own grandfather for her" . . .

"Poor Little Rich Girl" stars Farrah Fawcett as heiress Barbara Hutton, who "shocked the world with her extravagant life style." From C. David Heyman's biography . . .

Still to come: the ABC mini-series!! . . .