Reality-TV star Heidi Montag filed Tuesday morning for separation, but not divorce, from husband Spencer Pratt, a fellow star of MTV's "The Hills."

Though another "Hills" star, Audrina Patridge, said she believes the move is a publicity stunt, an anonymous source told RadarOnline.com that "this is the real thing. Heidi would never take legal action unless it was legitimate. This was not a stunt."

The petition, filed at the Santa Monica Courthouse, cites irreconcilable differences as the reason for separation. Montag, who filed personally and without a lawyer, also checked the community-assets box that specifies

"There are no such assets or debts subject to disposition by the court in this proceeding," which . . . Radar said implied that the couple has a prenuptial agreement.

The move comes as Montag - without Pratt - reportedly prepares to shoot a new, unnamed reality show with friend and new roommate Jennifer Bunney, who had made appearances on "The Hills." According to TMZ.com, which first reported the separation, Montag's earnings become her own independent property beginning with the date of separation.

"She's not quite ready to file for divorce [but] she had great concern for her finances," a second source told Radar Tuesday. "Spencer has been spending what she considers to be her money."

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As a source had told the site last week, "Spencer blew through all of their money. They've been living on credit."

Pratt received his last "Hills" check, for $675,000, about two weeks ago, Radar said.

"The way that kid blows through cash," a source told the site, "I wouldn't be surprised if he's broke by the end of July."