header files for R packages

I have 6 or 7 nice constants (for example 1852 meters per nautical mile)
I would like to have available to 4 or 5 functions in an R package.   In
C this would just be a header .h file and I would just "include"  I am
stuck trying to figure out how to create something like a C header file
for an R package.  Any ideas?