Whenever I see the word costar I mentally pronounce it as cost-er. So I was happy to see that the writers for the Yahoo! front page chose to put a hyphen in the word. Voila! I could pronounce it:
Except that someone else disagrees (it happens a lot at Yahoo!):
Which is correct, costar or co-star? I don’t know. I’d have to consult a dictionary. Maybe Yahoo!’s writers should, too.