My fellow countryman,

In my case, I would send:

graduation schedule, official invitation letter from University, Financial Affidavit letter stating who is going to be responsible for their travel expenses, medical and accomodation expenses during their stay etc. The letter should also assure why they don't have any intention of staying here.

Besides all supporting documents that convince the visa consulate to issue the visa to your parents.

Hope this helps,

Good Luck.