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As Christie Brinkley took the stage to give the commencement address at Southampton High School on June 25, one of the 134 graduating seniors toted a sign reading, "I love you, Christie!" Even without that show of support, Brinkley knew she had at least a couple of fans in the audience rooting for her: Husband Peter Cook and 11-year-old son Jack. "They looked celebrity-like, very well put together but also like a family," says principal Nicholas Dyno. "Everything seemed normal."

It did until Brinkley, a devoted environmentalist, concluded her speech on global warming. As the couple were about to leave the auditorium, they were confronted by Southampton Village police officer Brian Platt, who—as her husband stood next to her—delivered some shocking news to the supermodel: Cook, Platt claimed, had been having a yearlong affair with his stepdaughter, 19-year-old Diana Bianchi, who had worked as Cook's personal assistant.

The encounter left Brinkley stunned about the state of her nearly decade-long marriage. A three-time divorcée (see sidebar), Brinkley once said that in Cook she had finally found her "Prince Charming" and recently told Good Housekeeping of their family life: "Peter is our rock ... [he's] the calm in the middle of our storm." But after verifying Platt's identity at the station where he worked, Brinkley, 52, say sources, took Jack and daughter Sailor, 8, to L.A. to be with her parents. (Brinkley's other daughter, Alexa Ray, 20, with ex-husband Billy Joel, is a rock musician playing gigs in the New York City area.) On July 11 Brinkley's publicist announced the couple had separated. "She has been extremely concerned about the impact of this situation on her children and felt it was very important to protect them and take them away for a little bit," says one of Brinkley's close friends. "She is totally shocked and just devastated."

As are observers of the glitzy Hamptons scene, who had grown accustomed to seeing the couple—who own at least four homes valued at nearly $60 million (see box)—lend their star power to scores of local events. Over Memorial Day weekend, for example, they turned out for a local fundraiser for Planned Parenthood; three weeks later they walked the red carpet hand-in-hand at the Southampton premiere of the new Ed Burns movie The Groomsmen. "This marriage has been the gold standard in the Hamptons," says R. Couri Hay, society editor for Hamptons magazine. But now that news of the affair is out, "everyone is just horrified," says a longtime friend of the couple's.

Why would Cook, 47, a successful architect who has built luxury homes for heiress Anne Hearst, entertainment lawyer Alan Grubman and other members of the Hamptons elite, risk having a fling with a teenager? One friend says that Cook had long chafed at being known as "Mr. Brinkley. That became his identity—being her husband." Adds another source close to the couple: "He was given a very difficult role of having to play second fiddle to her. Life revolved around trying to accommodate her. He was definitely at her beck and call."

With Bianchi, the roles were reversed. An aspiring singer who in 2003 won a talent contest at Amagansett's famed music venue Stephen's Talkhouse, Bianchi met Cook during the summer of 2004, when she was 17. At the time Bianchi was working at Stevenson's toy store in Southampton, a shop frequented by Brinkley and her husband. "He's the best dad," says store manager Susan Watson. "He always comes in to buy his daughter things to surprise her."

That wasn't the store's only attraction. After Cook met Bianchi, "he used to come in a lot more than usual, I guess—that's what people were telling me," she said in an interview this week with Fox 5 New York, adding that when Cook offered her a job at his architecture firm as his personal assistant, with a salary that rose to nearly $50 an hour versus the $12 an hour she had been making, "I didn't think twice about it."

Within two weeks, however, Cook had begun making advances, says Bianchi's lawyer, Joseph Tacopina, who is investigating a possible sexual harassment lawsuit: "He said to her, 'How would you feel if I were attracted to you?'" Eventually, the two began an affair, and Tacopina claims that Cook offered to help Bianchi with music contacts, in addition to presenting her with money for a down payment on a Nissan Maxima and cash gifts that totaled as much as $2,000 a month. (Cook had no comment and neither did his new attorney Kathleen McKenna.)

When Bianchi tried to end the affair, Tacopina says, Cook continued to pursue her. Even after Platt, Bianchi's stepfather, went to Cook and asked him to break off the affair, he refused—which is why Platt decided to go to Brinkley herself. Says Tacopina: "What Cook did is prey on an 18-year-old girl. It's a textbook case of why an employer is not supposed to carry on a relationship with an employee."

After word of Cook's affair with Bianchi became public, another former employee, Samantha Cole, came forward to say she also become romantically involved with the boss a decade ago. She describes a similar pattern: He hired her, then pursued her. She was also 18 and a singer. Cole claims Cook, then 36, proposed marriage to her in May 1996—she declined—three months before he became engaged to Brinkley. "He was an old-fashioned romantic," says Cole, now 30, still friends with Cook. "The fact that [Bianchi's] story was so similar to what happened between us is strange. But he's married now, so there's a whole other issue. There are kids involved."

Ironically, it was, in part, Cook's desire to build a home life around children that convinced Brinkley to marry him after being fixed up with him by her pal, Today show reporter Jill Rappaport. After their September 1996 wedding, Cook raised Jack (whose biological father is Brinkley's third husband, Ricky Taubman) as his own, and the couple welcomed Sailor in 1998. The family spent many hours together at soccer and baseball games and going sailing and swimming. "I finally have what I always wanted—a husband who is so involved in the family, who cares about it with every ounce of his being, who is there for us," she told Redbook in 1999.

Now Cook may lose his family over what one of his friends calls "a stupid mistake." On July 15, four days after news of the split hit, a familiar scene unfolded in the Hamptons: Cook attended a local event, the annual Hamptons Designer Showcase, which featured an 18,000-sq.-ft. house his firm had designed. With his gold wedding band still on, Cook showed up looking "handsome, tan, his hair in that perfect beachy, preppy coif he always has," a fellow guest says.

There was, however, one notable difference: Brinkley and the kids weren't at his side. And although some close friends are hoping Brinkley and Cook can work things out ("They are both good people," says Christian Currey, who is a longtime friend of Cook's), most believe that their joint Hamptons reign has ended. "I am really shattered—they seemed to be an excellent team," says pediatrician Helen Caldicott, who worked with them on projects for the nonprofit Standing for Truth Against Radiation.

Still, those closest to Brinkley believe that she will come out ahead. "Peter was always very quiet—she was the one with the big personality," says someone who has known Brinkley for years. "She's so charming." And for better or for worse, she is experienced at this kind of thing too. "Christie has been through lots of difficult things but has had a wonderful life at the same time," says another pal. "This too shall pass."

  • Contributors:
  • Kathy Ehrich Dowd/Southampton,
  • Jeffrey Slonim/Southampton,
  • Lesley Messer/Southampton,
  • K.C. Baker/New York City,
  • Nina Burleigh/New York City,
  • Sharon Cotliar/New York City,
  • Liza Hamm/New York City,
  • Barbara Sandler/Chicago,
  • Beverly Keel/Nashville.