[R] Possible or not possible: serif axis labels with plotmath [but everything else sans serif]?

[Hofert Jan Marius]

Dear expeRts,

I it possible to have serif labels in the following plot?

x <- 1:10
y <- x
plot(x, y, type="b", xlab=expression(x[1]), ylab=expression(x[2]))

I know that one can use pdf(, family="serif"), but then also the axis tick marks 
are printed in serif font. Apart from the fact that it may not look nice, I'm 
just interested if one can have serif axis labels but everything else in sans serif
(default).

Cheers,

Marius