In their comprehensive reference The Cacti of Eastern Brazil (BEC 2004), Taylor & Zappi catalog Arrojadoa rhodantha as a strictly singular species (sensu stricto) consigning all previous listed species, subspecies and varietal forms to synonymy. Just for reference, I am posting here photos of plants I obtained under various specific and varietal names (not to say that are especially valid). I will add additional photos from time to time.
Arrojadoa rhodantha ............... (sensu stricto)
Arrojadoa theunisseniana ...............
Arrojadoa canudosensis ...............
Arrojadoa horstiana ...............