Are these authentic Cesca chairs or knockoffs?  

This design has been plagiarized a lot. I don't know how to tell the difference other than really obvious stuff, so I simply googled "authentic Cesca chair" and this link to a 1991 article popped up first:
http://www.nytimes.com/1991/02/07/garden/the-many-lives-of-a-very-common...
You can compare their points to your chair in person more easily than I can to your photos. Have fun!

Or you could just look at the chairs sold by Knoll and compare their photos, which I just did and now it's quite obvious that yours are not authentic. Probably not worth getting the cane redone.
(If it makes you feel any better, I bought a set of 6 at auction many years ago for a few dollars and of course they were fakes! They ended up rusting to death outside before I finally junked them.)

Thanks spanky! That's a nice article. I figured they were knockoffs, but for $30 I won't feel bad leaving them out in the garage until we have enough people over that we need extra chairs. A guy I know has an authentic chair that he tried to sell me a few months back but I wasn't really into it so I didn't look too closely at it. Anyway, thanks again!