The producers of hit medical drama ER are still "optimistic" they can persuade George Clooney to return for its final series.The show's executive producer David Zabel has already coaxed former stars Anthony Edwards and Noah Wyle back on set for the long-running drama's last series - and he now hopes to convince Clooney to make a cameo appearance.He says, "We have a really good storyline for every character from the past."But Clooney's spokesman has cast doubt on an appearance, insisting the actor is "busy making movies".The representative adds, "He is on record as saying he is not coming back. It is something he has already done."

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