integer. In 1998 Hirschowitz asked whether a smooth rational surface $ X $ defined over the
field of complex numbers, having an anti-canonical divisor not nef and of self-intersection
zero, has $(-2) $-curves. In this paper we prove that for such a surface $ X $, the set of $(-1)
$-curves on $ X $ is finite but non-empty, and that $ X $ may have no $(-2) $-curves. Related
facts are also considered. References