Tennant and Piper form "close friendship"

David Tennant and Billie Piper have formed a "close friendship" while filming the second series of Doctor Who together.

The pair were spotted holding hands and running down the street for scenes being filmed in Cardiff this week.

"They were having a great time," one onlooker told the Daily Star. "They looked like a couple of naughty schoolchildren."

Another source added, "They were giggling and flirting. It definitely looked like they were keen on each other."