I don't know the offset, but they are 18x8" in front and 18"x9" in the rear. I know that many 986 owners are running these wheels successfully, for instance:
FYI for anyone on the edge trying to decide on this deal...that's my car in the link just above and have been running them for just over 2 years now and LOVE them.
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"You finally got that Nazi-go-cart you always wanted." -Peer