uncompressed saves warning

[Env:  Windows XP Pro / R 2.14.1 / StatET / R-Forge]

A package of mine now generates a Warning under R 2.15.0 beta on CRAN 
checks:

* checking data for ASCII and uncompressed saves ... WARNING

   Note: significantly better compression could be obtained
         by using R CMD build --resave-data
                 old_size new_size compress
   gfrance.rda      300Kb    179Kb       xz
   gfrance85.rda    295Kb    176Kb       xz

What is the equivalent R command to compress these files in my project tree?

Michael Friendly     Email: friendly AT yorku DOT ca
Professor, Psychology Dept.
York University      Voice: 416 736-5115 x66249 Fax: 416 736-5814
4700 Keele Street    Web:   http://www.datavis.ca
Toronto, ONT  M3J 1P3 CANADA