External Referees

I want to have an american expert as referee. However, the american professor cannot attend my defence. Is it possible to install that expert as an (additional) referee?

Referees have to be members of the PhD committee.
It is necessary that all members of the PhD committee personally attend the defence (§14 of the PhD regulations). Attending the defence via video conference is not sufficient.
Even if the committee has more than two referees, all members have to attend the defence. It is not possible to reduce the committee after it has been installed by the Council of the Faculty (FBR).
Conclusion: The PhD regulations currently do not allow an (additional) referee who will not attend the defence.

PhD in English

I want to have an external referee who doesn't speak German. Can I write my PhD thesis in English? The defence has to be in English, too.

§7 of the PhD regulations state that the PhD thesis has to be written in German. The Council of the Department (FBR) may allow other languages. The regulations do not have a statement about the language of the defence.
In 1984, the FBR decided that English is generally allowed for PhD thesis.
The defence may also be in English if a committee member does not speak German and the whole committee agrees that English is spoken. In that case, the talk should also be in English.

Restrictions for inspection of the thesis and the defence

Does the department has restrictions regarding the date when the inspection of the thesis and the defence are possible?

Yes. In 1992, the professorate of the department agreed on following restrictions which still are valid:

• No inspection and defence after end of lecture period in summer until end of August.
• The same is valid for the periods of Christmas and Easter vacations.
• No defence during the conference month September.
• Inspection period can start only in September. The following defence is possible not before end of September/begin of October.