British actress Sienna Miller has slammed reports she is cheating on her boyfriend Rhys Ifans with Hollywood star Sean Penn. Miller and the Mystic River star - who is divorcing wife Robin Wright Penn after 11 years of marriage - were recently spotted cosying up at a New York hotel, reports the New York Daily News. But the Layer Cake star has branded the reports "outrageously untrue" and insists the pair are just friends. A spokesman for Miller says, "Sienna is a very good friend of Robin. She adores and respects Robin."