  We probe directly the superconducting current-phase relations (CPR) of
Nb/Au/(001) YBa2Cu3Ox heterojunctions prepared in epitaxial c-axis oriented
YBa2Cu3Ox (YBCO) thin films. The measurements are performed by a
single-junction interference experiment. Atomic Force Microscope observations
and estimations made from I-V characteristics confirm the c-axis current
transport. No deviation of the current-phase relation from a sinusoidal shape
has been found within the experimental accuracy. Possible reasons of the
absence of the second harmonic in the CPR due to a second order tunneling are
discussed.
