D0-D-bar 0 mixing in the presence of isosinglet quarks.

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