  In this talk we report on a study on the mixing of neutral charmed mesons and
argue that, at the present stage, a CP violating weak phase of the order of
some per mille can not be excluded in the Standard Model. It is shown how some,
seemingly reasonable, simplifying assumptions about CKM couplings lead to the
wrong conclusion that CP violation of this amount is an unambiguous indication
of new physics. The presented results rely on a recent short-distance analysis
of the Delta C=2 transition, which confirms the expectation that the dominant
contribution is due to effects of flavour symmetry breaking appearing in higher
orders of the Heavy Quark Expansion. We investigate meson-antimeson transitions
with an intermediate state coupling to the meson's sea quark background,
present in dimension 10 and 12, using a factorisation approach to simplify the
operator basis. On account of a lifting of GIM suppression by one power of
m_s/m_c, the contribution to y=Delta Gamma/2 Gamma is found to exceed that of
the formally leading dimension six by a factor close to ten.
