There is no hard and fast rule. Indeed many WG chairs have work or personal interest in the area of the group (how could it be otherwise?) and occasionally write drafts. However, the preference is not to be an author. In particular, if you are an author you should not be the chair deciding or document shepherding about the topic. This is one reason why we have multiple chairs. In general, it is easy to get into a situation where it is difficult to progress a document because all the parties who should do something about the document are recused due to being authors. Having other people hold the pen and the chairs doing their chair job is a far better model. Jari