None. Or if they have, it's nothing we've yet been able to recognize as writing.

I've excavated many civilizations, in many foreign lands, but none so foreign as an alien planet. I find myself making the most basic assumptions about them based on what I have always known--assuming they have two eyes, that they have male and female--and then catch myself and remember that all other mammals we've seen on this planet have four eyes and are hermaphroditic, and perhaps these people were as well. So I think we have not found any writing, but perhaps their way of writing is simply beyond my understanding and requires four eyes to see.