reftex-cite-punctuation

reftex-cite-punctuation is a variable defined in `reftex-vars.el'.
Its value is

(", " " \\& " " {\\it et al.}")

Documentation:
Punctuation for formatting of name lists in citations.
This is a list of 3 strings.
1. Normal names separator, like ", " in Jones, Brown and Miller
2. Final names separator, like " and " in Jones, Brown and Miller
3. The "et al" string, like " {\it et al.}" in Jones {\it et al.}

You can customize this variable.