  We report a new class of $c$-axis Josephson tunneling experiments in which a
conventional superconductor (Pb) is deposited across a single twin boundary of
a YBa_2Cu_3O_{7-\delta} crystal. We measure the critical current as a function
of magnitude and angle of magnetic field applied in the plane of the junction.
In all samples, we observe a clear experimental signature of an order parameter
phase shift across the twin boundary. These results provide strong evidence for
mixed $d$- and $s$-wave pairing in YBCO, with a reversal in the sign of the
$s$-wave component across the twin boundary.
