  The gluon polarisation in the nucleon has been determined by detecting charm
production via D0 meson decay to charged K and pi in polarised muon scattering
off a longitudinally polarised deuteron target. The data were taken by the
COMPASS Collaboration at CERN between 2002 and 2006 and corresponds to an
integrated luminosity of 2.8 fb^-1. The dominant underlying process of charm
production is the photon-gluon fusion to a cc-bar pair. A leading order QCD
approach gives an average gluon polarisation of (Delta g/g)_x= -0.49 +-
0.27(stat) +- 0.11(syst) at a scale mu^2 ~ 13 (GeV/c)^2 and at an average gluon
momentum fraction (x) ~ 0.11. The longitudinal cross-section asymmetry for D0
production is presented in bins of the transverse momentum and the energy of
the D0 meson.
