  Using a sample of four million hadronic Z events collected in ALEPH from 1991
to 1995, the decays D*+ -> D0pi^+_s, with D0 decaying to K-pi+ or to K+pi-, are
studied. The relative branching ratio B(D0 -> K+pi-)/B(D0 -> K-pi+) is measured
to be (1.84+-0.59(stat.)+-0.34(syst.))%. The two possible contributions to the
D0 -> K+pi- decay, doubly cabibbo-suppressed decays and D0-Dbar0 mixing, are
disentangled by measuring the proper-time ditribution of the reconstructed
D0's. Assuming no interference between the two processes, the upper limit
obtained on the mixing rate is 0.92% at 95% CL. The possible effect of
interference between the two amplitudes is also assessed.
