Should I expect to be paid for post-graduation research? I graduated with my Master's degree in Computer Science a couple of years ago. For a graduate requirement, I had to take on a project of significant scope and write a paper about the project. I had continued my project after graduation in order to attempt to publish a paper. This unfortunately did not happen, as I entered the workforce and focused my time on my new employment. 

I received an email from my professor a couple of days ago asking if I had a copy of the source code to my project, because he has a new student who he wants to take over the project. He noted that if he was able to publish a paper with the new student, that my name would be added to the paper. 

The professor had asked if I wanted to participate in the project, I replied that I would be able to help work on it in hopes of upping the chances that we get the paper published.

Since I am putting in hours post-graduation: should I expect to be compensated for my further work on the project, should I expect that this work is strictly voluntary in order to get my name on a paper, or is compensation up to the professor in charge of the project?
You should ask the professor. However, in my experience, the default assumption is that you will be not paid (if not said otherwise). It is often hard (again, in my experience) for professors to obtain funds for short-term-positions.

Note, however, that you are under no obligation to finish the project without getting paid. So before working on this for free, think about if you have time for it and would profit from it in some way (e.g. if you like thinking about it or if a publication is good for your life/career).

So to make clear the answer to the question: Ask, but do not expect.