  We show that the Novikov-Shubin invariant of an element of the integral group
ring of the lamplighter group Z_2 \wr Z can be irrational. This disproves a
conjecture of Lott and Lueck. Furthermore we show that every positive real
number is equal to the Novikov-Shubin invariant of some element of the real
group ring of Z_2 \wr Z. Finally we show that the l2-Betti number of a matrix
over the integral group ring of the group Z_p \wr Z, p>1, can be irrational,
and so the groups Z_p \wr Z become the simplest known groups which give rise to
irrational l2-Betti numbers.
