D-0-(D)over-bar(0) mixing in the presence of isosinglet quarks

We analyse Delta C = 2 transitions in the framework of a minimal extension of the standard model where either a Q = 2/3 or a Q = -1/3 isosinglet quark is added to the standard quark spectrum. In the case of a Q = 2/3 isosinglet quark, it is shown that there is a significant region of parameter space where D-0-<(D)over bar (0)> mixing is sufficiently enhanced to be observed at the next round of experiments. On the contrary, in the case of a Q = -1/3 isosinglet quark, it is pointed out that obtaining a substantial enhancement of D-0-<(D)over bar (0)> mixing, while complying with the experimental constraints on rare kaon decays, requires a contrived choice of parameters.