Subject: Which fox species can hybridize with which (if any)?

I've had an interest in foxes for quite a while now.  As I've browsed the 
internet, I've occasionally come across someone who claims to have seen a "red 
fox / gray fox hybrid" at some time or another.  Of course, I myself know that 
the coloration of both species can vary considerably -- gray foxes can have a 
large amount of red, and red foxes can have a considerable amount of gray on 
them (indeed, silver red foxes are completely gray).  Thus, I assume that 
these "hybrid" conclusions are most likely misguided.  However, I am curious: 
Regardless of whether they normally do so in the wild, is it actually possible 
for a red fox and a gray fox to hybridize?  What about other foxes, such as the 
swift/kit (apparently considered to be the same species now), arctic, and 
fennec?

Re: Which fox species can hybridize with which (if any)?