German election: Who could succeed Angela Merkel as chancellor?

The campaign to replace Angela Merkel as German chancellor is wide open, but all of the contenders confront the same difficult task: how do you stand out when you're up against such a political behemoth?
As chancellor, Mrs Merkel has controlled German politics for the last 16 years. Before the federal election in September, her successors must create an impression.
Here's a brief rundown of who they are, along with an evaluation of their prospects from our Berlin reporter Damien McGuinness.