Christina Onassis and Sergei Kauzov denied yesterday that their 8-day-old marriage is in trouble. Onassis told United Press International by telephone from Athens that she will rejoin Kauzov in Moscow as soon as she completes some business affairs in Greece. And Kauzov in Moscow, said he talks to Onassis regularly on the phone.

The phone calls were vouched for by Soviet journalist Victor Louis, who wrote in a dispatch to the London Evening News that the newlyweds are chatting twice a day and that the conversations are often "passionate." Louis quoted Kauzov's mother as saying the calls prove Onassis is "crasily in love with my son."

The rumor that the young marriage was coming apart got started Saturday when Onassis canceled plans for a Siberian honeymoon and flew to Athens. But the trip is strictly business, the daughter of the late Aristotle Onassis told UPI.

"I'm coming back," she said. "I don't know the exact date because I've got things to do. But when I'm finished I'll be back."

Kauzov's alleged ties to the KGB, the Soviet security police, had been reported to have been a cause of the rumored marital rift, but Kauzov yesterday denied a KGB connection. "This is something I have been expecting," he said. "The newspapers are going to say some nasty things about me."