User blog comment:Simplyme/Fading?/@comment-71.205.210.226-20110909224855/@comment-3459279-20110909225450

Helios and Selene "faded" or "dissapeared" because the Romans combined their duties with Apollo and Artemis. Riordan only really mentioned them for a short time in the series (in book 3 I think) and otherwise doesn't mention them...they could be gone for good, they could be just chilling in the background