I have posted several photos of an apparent Elegant Tern - taken yesterday in Buenos Aires province, Argentina - on the ID discussion photo gallery. The nearest population is central Chile, so it would constitute an impressive record.
This is a potential first for Argentina, so it is important to eliminate other spp. Particularly with a taxon that has caused confusion when it has turned up as a vagrant.
To me, bill structure/shape/colour, extent of black crest and size rule out Royal and Cayenne, which are both regular/common here.