[R] use of "@" character in variable name

[Mike Miller]

Importing data with a header row using read.delim, one variable should be 
named @5HTT but it is automatically renamed to X.5HTT, presumably because 
the "@" is either unacceptable or misunderstood.  I've tried to find out 
what the rules are on variable names but have been unsuccessful.  I'll bet 
someone here can tell me where to look.  Maybe it's hidden away in here 
somewhere:

http://cran.r-project.org/doc/manuals/R-data.pdf

Thanks in advance.

Mike