[R] contour lines in windows device but neither in pdf nor in postscript

[Patrizio Frederic]

library(mvtnorm)
x     = seq(-4,4,length=201)
xy    = expand.grid(x,x)
sigma = (diag(c(1,1))+1)/2
d2    = matrix(dmvnorm(xy,sigma=sigma),201)
xsamp = rmvnorm(200,sigma=sigma)

contour(x,x,d2)
points(xsamp,col=3,pch=16)

pdf("pdftry.pdf")
contour(x,x,d2)
points(xsamp,col=3,pch=16)
dev.off()

postscript("pstry.ps")
contour(x,x,d2)
points(xsamp,col=3,pch=16)
dev.off()

# I can see contour lines in a window device but I can't see them in
files pdftry.pdf and pstry.ps
> version
               _
platform       i386-pc-mingw32
arch           i386
os             mingw32
system         i386, mingw32
status
major          2
minor          7.1
year           2008
month          06
day            23
svn rev        45970
language       R
version.string R version 2.7.1 (2008-06-23)

what's going wrong?

Thanks in advance,
Regards,

Patrizio Frederic