Edit: after research, I can now say that this is a female black redstart :)
Just below is a picture I found to illustrate female black redstart
And here is my visitor:
This is a new bird I've first noticed yesterday. There are actually two of them from the same breed (I think). This is probably the female. The second one is very similar, except it is darker and the orange tails stands out more. The male also has some white on top of the wings. It's not obvious on this picture, but the tail is pretty red/orange, about the same colour as the plastron on red robins.
Anybody knows what they are? I didn't find them in my book about European birds...