Photoluminescence investigations on undoped n-type GaN layers grown on 6H-SiC and sapphire reveal the presence of residual acceptors with a binding energy of 230 meV. Their presence in high temperature vapor phase epitaxy grown layers is strongly correlated with the graphite susceptor containing the Ga. Mg as a contamination can be ruled out. In metal organic vapor phase epitaxially grown layers, the metal organic are probably the source of the carbon contamination. It is concluded that carbon on nitrogen sites introduces the most shallow acceptor in GaN. The experimental observations are supported by an estimate of the acceptor binding energy using effective-mass-theory.