[R] transparent grays?

[Frostygoat]

Is there a grayscale equivalent to alpha levels in rgb?

Example:  I have the following to make red transparent circles overlap
with previously plotted blue symbols.

symbols(x=sites$long,y=sites$lat,circles=log(sites$prop.nem
+1),add=T,inches=F,bg=rgb(red=1,green=0,blue=0,
alpha=0.5),fg=rgb(red=1,green=0,blue=0, alpha=0.5))

I'm having a hard time coming up with a grayscale equivalent.

Thanks!