R help - Mar 2011 - expression at labels in contour function

Hello,

I'm stuck with the following code.

p <- 0:100/100
a <- 0:200/100
beta <- matrix(a + p%x%(1-a), nrow=length(a), ncol=length(p))
lb <- c(expression(beta==0), expression(beta==0.2), expression(beta==0.4),
expression(beta==0.6), expression(beta==0.8), expression(beta==1),
expression(beta==1.2), expression(beta==1.4), expression(beta==1.6),
expression(beta==1.8))
contour(a, p, beta, labcex=1, labels=lb)

Does anybody have an idea on how to make the greek letters appear
at the equi-\beta curves?

I'm running R version 2.11.1 on a 32-bit Windows 7 Starter and on a
Windows Vista Business without success.

Thanks in advance,

Rogerio