I graduated 8 months ago with 7 SCI article published. Recently, I submitted another article based on my thesis work. My supervisor, who actually has no supervisory role in my thesis except for providing a monthly stipend, wants me to add him as a co-author. He argues that he supported my research, but in fact I paid him back more than he supported me, in the form of my work on various projects (totally different than my thesis topic).
During my PhD he never took an interest in my PhD thesis, rather he always used me for his short term industry projects. In 4.5 years we didn’t have a single discussion/meeting on my PhD thesis.
Is him asking me to add him as an author morally correct?
What if I refuse his request? Can my action be consider an unethical act?