Imagine if bookish young witch Hermione Granger had married baddie Draco Malfoy instead of good friend Ron Weasley at the end of the Harry Potter movies (2001 to 2011).

English actress Emma Watson and actor Tom Felton again sparked speculation that they were dating when he posted a photo of them together on Instagram on Monday (Aug 19).

They play Granger and Malfoy in the film series, with Weasley played by Rupert Grint and Harry Potter played by Daniel Radcliffe.

In the picture posted by Felton, 31, with the location stated as South Africa, he is seen teaching Watson, 29, to play the guitar and the former co-stars are in pyjamas. He captioned the photo: "Quick learner x."

The two first sparked rumours last year that they were dating, when Watson posted a photo of her and Felton at a beach and a video of them skateboarding together in November.

In February this year, she posted a photo of herself taken by Felton, as she wrote, "Friends capture you best" and tagged him in the caption.

The pair have maintained through the years that they are good friends, although they have appeared frequently in each other's Instagram posts.

Watson revealed to the media in 2011 that she had a crush on Felton when she was younger, but Felton said he viewed her like a younger sister.

Watson will star in a new movie, Little Women, later this year, while Felton is in the film Ophelia, which was released in June.