levels update

Hello,

I hope this question is not too stupid. I would like to know how to update
levels after subsetting data from a data.frame.

df <- data.frame(factor(c("a","a","c","b","b")), c(4,5,6,7,8), c(9,1,2,3,4))
names(df) <- c("X1","X2","X3")

my.sub <- subset(df, X1 == "a" | X1 == "b")
levels(my.sub$X1)

# still gives me "a","b","c", though the subset does not contain entries
with "c" anymore

I guess, the solution is rather simple, but I cannot find it.