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The star of this year's biggest box-office hit is now turning his sights to YouTube's subscription service.

Robert Downey Jr is following up the Avengers movies with an online series about artificial intelligence, scheduled for 2019.

The Iron Man actor plans to present the eight-part documentary, as well as co-produce it with his wife, Susan Downey.

One expert said the move gave the YouTube Red pay service a boost ahead of its planned global expansion.

"If you look at all of YouTube Red's original stuff, most of it is people who became famous on YouTube," Tom Harrington of Enders Analysis told the ����ý.

"This is just them diversifying. It gives the whole product offering a look that isn't a bunch of kids talking at a camera and pulling pranks, which is the dismissive attitude that a lot of people take to vloggers."

'It's about perception'

YouTube Red first launched in 2015 as an answer to other paid subscription services such as Netflix, Amazon Video and Spotify. Subscriptions cost $10 (£7.42) per month.

It is currently available only in the US, Australia, Mexico, New Zealand and South America, but the company has said that it plans to launch in more countries this year.

Mr Harrington said that working with stars such as Robert Downey Jr was a way of expanding the platform beyond its original scope, which focused on monetising personalities who became famous on YouTube.

"It's about perception of what's offered, and they're obviously looking to shift that," he told the ����ý.

"Suddenly you've got a show with Robert Downey Jr and AI and you think, 'Maybe I've got YouTube Red wrong, maybe I need to look into this.'"

The announcement of the AI series follows the successful run of another YouTube Red original. Cobra Kai, which is based on the Karate Kid franchise, topped for digital originals last week.

Eight days after the series' debut this month, YouTube announced that it would be renewed for a second season.