[R] determing font type in expression

[Thomas Hoffmann]

Dear R-Maillist members,

I would like to label the y-axis of a plot with a mathematical 
expression "sed.rate mm a-1", where the "-1" is printed subscript. I am 
using font.lab=15 for the x-axis. However, when plotting the y-axis with 
the expression-function it appears in a different font. Does anybody 
knows how to use font.lab=15 in the expression as well?

Cheers
Thomas