  We show that the study of CP asymmetries in neutral B-meson decays provides a
very sensitive probe of flavour-changing neutral currents (FCNC). We introduce
two new angles, $\alpha_{SM}$ and $\beta_{SM}$, whose main feature is that they
can be readily obtained from the measurement of the CP asymmetries $a_{J/\psi
K_s}$, $a_{\pi^+ \pi^-}$ and the ratio $R_u
\equiv|V_{ud}V_{ub}^*|/|V_{cd}V_{cb}^*|$, providing a quantitative test of the
presence of new physics in a model-independent way.
  Assuming that new physics is due to the presence of an isosinglet down-type
quark, we indicate how to reconstruct the unitarity quadrangles and point out
that the measurements of the above asymmetries, within the expected
experimental errors, may detect FCNC effects, even for values of $|\sum_{i=1}^3
V_{id} V_{ib}^* / (V_{td} V_{tb}^*)| $ at the level of a few times $10^{-2}$.
