Whenever I open an RStudio project it also opens Microsoft Publisher

Whenever I open an RStudio Project (e.g. by double-clicking) it not only starts RStudio (or switches to the project when opening a project from inside RStudio) but also starts Microsoft Publisher, which then of course fails because Publisher cannot open the file.

I'm not sure why this happens and how to resolve this so that Publisher does not start anymore. Any help appreciated!