Herbarium plants are sometimes a clue though Myriam, by seeing something of similar habit can help.
I can't remember what I searched for but I found another species with 'orbiculata' as the species, Chamaecrista orbiculata and I found that by seeing a similar habit in a herbarium photo. It's not your plant, but the leaves again are the same shape as Cotyledon orbiculata so there's a real chance it could be Zornia orbiculata, I used the 'find' on the page feature and it's the only other genus in Fabaceae in Brazil with that species name. There's 6 species of Chamaecrista with 'orbiculata' in the species name too as subspecies, it looks more of a woody plant but remember the tree in the nettle family! The flower shape looks a good possibility too.
http://www.flickr.com/photos/m...
