Is it common to claim co-authorship by helping writing a paper without doing any research In my research group, there is a researcher who usually approaches other PhD students who are about to write papers in order to offer his help. Normally, he does not actually do any research related to the paper. Instead, he reads our papers, makes comments about the writing style and sometime re-writes some sections in order to make them more readable. At the end, he will claim the co-authorship of the paper. I can see it is very helpful for new PhD students who don't have much experience in writing papers or articles. However, it is sometime uncomfortable for me to see someone who doesn't do actual research but still manages to get an authorship.

So, I want to ask if it is a common practice for someone to help writing a paper without doing any actual research relating to it, and claim the co-authorship. If it is not, how should I react if someone wants to do the same to me?
people who help write papers may help fulfill one of the pillars of scientific discovery... communication of the science.


There is an appropriate place to credit people who read your paper and offer useful comments on it: the acknowledgement section. It is very common to see acknowledgements "for providing valuable feedback", "for suggesting a cleaner presentation", "for pointing out important related work" and so on. Even "for providing a simpler proof of Lemma X.y". 

None of this rises to the level of co-authorship. 

What's worse in this case is that 


  At the end, he will claim the co-authorship of the paper. 


While I'd find the idea of co-authorship for such contributions odd, I would not think too much about it if it were negotiated in advance (as is the theme of many of the answers on this site). But to offer what appears to be unconditional help first and then (when the student really has no choice in the matter)  to demand co-authorship is plain wrong. 

To answer your last question on what to do, the answer, as is always the answer, is to negotiate things up front. It's a little awkward, but a little pre-collaboration awkwardness is MUCH better than a lot of post-collaboration recrimination and hostility.