There is also gamma (lim_{n -> infinity} (sum_{k=1..n} 1/k)-ln n), that, as far as I know, hasn't been proved to be either rational, either irrational, that is

There is also gamma (lim_{n -> infinity} (sum_{k=1..n} 1/k)-ln n), that, as far as I know, hasn't been proved to be either rational, either irrational, that is "long suspected irrational", and probably is.