External committee members for European PhDs

In our most recent "how can we help you?" thread, European PhD candidate writes,

I am currently a PhD candidate at an European university, and I wonder what exactly the relationship between the candidate and the external members of their committee is.
What are exactly the tasks of the external committee members?
How soon should one invite an external professor to be member of their committee?
Can one ask the external members of their committee to write letters of recommendation on their behalf?
Thank you in advance!

This is a great query. Unfortunately, given that I've spent my entire career in academia in the US, I don't know anything about it. But I'd be very curious to learn more and help European PhD candidate and others in their position out. Anyone with experience here have any helpful answers?

Comments

[Anon]

I have a PhD from a European institution that relied heavily on external members, since it was based in a scientific institution with very few philosophers working full time and permanently in it. The PhD program was 4 years, and at the end of the first year I asked an American scholar if he wanted to be my 'external advisor'. His duties were merely to read a report on my progress (and sign it) and, if he wanted, to help me out with my research more thoroughly (like commenting on my papers, etc). I spent a year at his institution, and he still writes me letters of recommendation. He was not my PhD supervisor, but I have to admit that he read almost all papers that composed my dissertation, and he gave me really valuable suggestions.
My colleagues were not as lucky as I have been

[Another European]

European student here as well.
I am curious about how it happens in the US. When should one invite the external members of their committee? Is it fine to ask for letters of recommendation?

[Postdoc19]

Re: "what exactly the relationship between the candidate and the external members of their committee is"

In my case (a European PhD), much of the information I needed on matters of this sort was spelled out in the program's regulations. So if this candidate has access to the regulations governing his/her own program, I would start there, as the details of the relationship might be program specific. (E.g., I had an external member on my examination committee who was appointed to me involuntarily and with whom I was forbidden contact prior to my defense. I also had a scholar from within my institution but from outside my subfield involuntarily appointed to my supervisory committee and with whom I was required to have contact prior to my defense. Yet I don't think these practices hold for all European programs.)

[JS]

This is difficult to answer, since the requirements vary a lot across Europe. I would advise to ask the department's chair and/or your supervisor. I did my PhD at the University of Vienna (and I am now Assistant Professor at an American University). The University of Vienna distinguishes between the role of the supervisor and the role of the reviewers. I had two supervisors and two reviewers. One supervisor and one reviewer had to be external. They had the same duties like the internal supervisor and the internal reviewer. I could choose the supervisor myself and approached her as soon as possible. She naturally wrote (and still writes) letters of recommendation for me. The external reviewers were appointed by PhD committee of the department, after I had finished my dissertation. I could propose reviewers, but had no influence on the final choice (I do not remember whether I had a veto). My external reviewer proposed to write letters of recommendation for me, but I never asked him. I believe that a letter from an external reviewer would weigh more than the letter of a (external) supervisor.

[PK]

Practices vary by institution, so you should ask your supervisor. At my institution, two external reviewers are required after your dissertation is written. It's the supervisor, not the PhD-to-be, who asks them to review. One of these reviewers will also serve as opponent in your public PhD defence. While most supervisors here are delighted if the PhD-to-be can suggest possible reviewers (or even approach them informally), ultimately, both the decision on who to take on as a reviewer and the responsibility for finding suitable reviewers lie on the supervisor.