After a phone interview in mid-January, Dr. X sent me an email (the same day) and offered me his advertised Ph.D. assistantship. I accepted via email and was then told to officially apply to the University (he pre-selected his student rather than have everyone waste money and time officially applying beforehand). Although I have yet to receive an official acceptance letter, in both our minds I'm going to be his student. Another school invited me to attend their interview weekend. I'd rather not go and let someone else attend, but I fear something going wrong with my number one choice. What are the chances of something happening that's out of his control (e.g. with the department or the university)? Should I email him and ask, or go ahead and attend the interview weekend at the other school just in case? I guess I would feel better if I had something official. Has anyone else been in the situation of having been unofficially accepted?