  The angular dependence of the nonlinear transverse magnetic moment of
untwinned high-quality single crystals of optimally doped YBCO have been
studied at a temperature of 2.5K using a low frequency AC technique. The
absence of any signature at angular period 2\pi/4is analyzed in light of the
numerical predictions of such a signal for a pure d-wave order parameter with
line nodes. Implications of this null result for the existence of a non-zero
gap at all angles on the Fermi surface are discussed.
