Claus von Bu low's ex-mistress, Alexandra Isles, whose testimony helped convict von Bu low at his first trial in 1982 on charges that he twice attempted to kill his heiress wife, yesterday made a last-minute dramatic arrival at Boston's Logan Airport. She is expected to testify today as a prosecution witness at von Bu low's retrial in Providence, R.I.

Isles' appearance came one day before a deadline set by the trial judge, who indicated to prosecutors that they would have to produce the former soap opera actress in person if they wanted to use her testimony. Prosecutors failed to convince the judge that they should be allowed to use a transcript of Isles' testimony from the first trial.

She arrived yesterday afternoon on a flight that originated in Frankfurt and that stopped in London.

Northwest Airlines stewardess B.A. Johnson told the Associated Press that Isles sat in the upper-deck lounge for the flight and was "real quiet. She didn't eat. None of us knew who she was."

Another flight attendant, who did not give her name, said, "The whole time she was pensive. She slept most of the flight. She had sodas."

Isles, 39, wearing a light blue dress, dark blue scarf and red-frame sunglasses, was met by Rhode Island Assistant Attorney General Marc DeSisto and Rhode Island state police Lt. Jack Reise, along with several Massachusetts troopers.

Isles, who was reported sequestered in a Boston hotel last night, told the jury at that first trial that she had given von Bu low an ultimatum to divorce his wife, multimillionaire heiress Martha (Sunny) von Bu low, and marry her within six months. That deadline fell near the time of the first of two comas the state says Sunny von Bu low suffered allegedly after her husband injected her with insulin in December 1979. She fell into a second coma in 1980 from which doctors say she will not recover.

Friday night, federal officials had issued a warrant for Isles on a charge of fleeing to avoid testifying. Those officials believed she was traveling in Europe and was unwilling to testify.

Von Bu low's chief defense lawyer Thomas P. Puccio said yesterday after he learned of Isles' return to the United States, "I was skeptical about her showing up, but when I read that the prosection had filed federal criminal charges against her . . . it did not surprise me she showed up after that." According to Puccio, that charge carries a penalty of five years in jail and a $5,000 fine.

The pursuit of Isles outside the courtroom in Providence is part of a running sideshow where the media are as captivated by the out-of-court developments as they are by the trial.

Andrea Reynolds, 47, the woman who is von Bu low's current love interest, sits every day in a van at the foot of College Hill in Providence watching the trial on a console of television monitors. The van is rented by Cable News Network, which is beaming the trial across America.

Reynolds, a thrice-married Hungarian-born grandmother, is banned from the courtroom across the street, technically because all potential witnesses -- of which she is one -- have been told by the judge to stay away so that their testimony is not tainted. Watching the trial on television would seem to negate the intent of that rule, but Reynolds is being left alone.

Perhaps more to the point, defense lawyer Puccio says, "I have been certified as sane by three doctors and I don't usually have the girlfriends of accused murderers in court when they are on trial."

Von Bu low, who meets with Reynolds after each day's court session, is still wearing a gold wedding ring.

During recesses in the court proceedings, von Bu low steps into the corridor, taps a Vantage filter cigarette into his hand and holds forth on a variety of idle topics from the filming of "The Great Gatsby" at Newport, where he lived with Sunny in the Clarendon Court mansion, to the quality of the pizza the press corps ate the night before.

The would-be made-for-television movie has already been cast by some of the irreverent scribes who sit through the trial every day. Von Bu low will be played by Tony Burton, the reporter for the New York Daily News, who bears a spooky resemblance to the defendant. Sunny von Bu low will be played by the mysterious blond who sits in the gallery every day, her hair pulled up in a pompadour.

Both von Bu low and Reynolds have an annoying habit of bouncing on and off the record, mostly off, which got Reynolds into a bit of a mess last week.

A roving People magazine reporter caught up with Reynolds and in her melodic but undefinable European accent she let loose with some quotes on, among other things, Rhode Island ("the most corrupt state in the nation") and Puccio (the jury does not like him, she said; "you can tell by their body language").

Reynolds insists that the conversation was off the record but People says no dice. It is unclear whether the incident will deter the couple from their regular practice of inviting select groups of reporters to join them at dinner -- off the record, of course.

Judge Corinne P. Grande often makes off-the-record comments to reporters herself in the courthouse hallway. Last week she stepped outside her chambers and answered reporters' questions at some length, an unheard-of practice in most courthouses.

There is a "von Bu low Media Center" on the 15th floor of a fancy office building a couple of blocks from the courthouse. The major networks are paying as much as $2,000 a month for air-conditioned "editing suites." Without a monthly contract, a phone and a seat cost $100 a day.

The New York Times is here, as well as the wire services, a phalanx of Rhode Island reporters, and even Vanity Fair and at one point Der Spiegel. But it is the tabloids who are having a field day.

"It's got sex, money and high society and the big question . . . did he or didn't he?" said New York Post reporter Joe Nicholson, who also covered the first von Bu low trial.

"I look for revenge, betrayal, love, lies and a man falsely accused, if that happens to be the case," said Nicholson during an interview at the Biltmore Plaza, which is the address for anyone who is anybody at the von Bu low trial.

"This trial is not a matter of substance," he said as he packed to go home for the weekend. "I guess it's a phenomenon of voyeurism into the lives of the rich.