I am preparing the presentation of a co-authored research, who owns the copyright?
German copyright is very different from UK or US copyright. In German law, copyright always belongs to the author(s), and cannot be given away. What you can give away are the various rights of use.

The question here thus should be whether your university has a right to use your presentations, potentially even an exclusive right to use them. In the case of computer programs, §69b of the German copyright law explicitly states that the employer has the exclusive right to use the work.

I am not sure whether a lawyer could argue that your presentation is a computer program - maybe he could if it is a tex file :). Otherwise, as F'x writes, it depends on your work contract. If there is not an explicit statement in your work contract that the university has the right to use any copyrighted works that you authored during your employment, they will generally not have such rights. In the typical employment contracts at German universities, such statements are not included.