philwelch is right on, but to give a less technical explanation, it's a signal to pronounce the world (co)(operating) instead of (coop)(erating). The New Yorke

philwelch is right on, but to give a less technical explanation, it's a signal to pronounce the world (co)(operating) instead of (coop)(erating). The New Yorker does it for all doubled vowels that aren't pronounced as long vowels.