Real simple: Beta! Often called "phage beta" b-prophage is a more modern way to write it. I knew that one off the top of my head, but I like to refer the general public interested in bacteriology to Tadar's online text.
It's nicely done, and I wish that it had been around when I was studying bacteriology.
Specific to your question:
> Hence, Corynebacterium diphtheriae can only produce the toxin responsible for the disease if it carries a temperate virus called phage beta. Only lysogenized streptococci produce the erythrogenic toxin (pyrogenic exotoxin) which causes the skin rash of scarlet fever; and some botulinum toxins are synthesized only by lysogenized strains of C. botulinum.