Is the gap anisotropy in high-T sub(c) superconductors really as high as it is commonly believed?

Making use of the photoemission data on the dependence of the order parameter Delta on the in-plane momentum k and of the tight-binding fit to the Fermi surfaces in hole-doped high-T sub(c) superconductors, we have calculated the anisotropy parameter Chi which enters into the formula for T sub(c) versus impurity concentration in an anisotropic s wave superconductor. We have found that Chi is as low as approximately 0.1 in a wide range of hole doping and for different sets of model parameters adjusted to describe the experimental observations of Delta (k) and Fermi surface in Bi sub(2)Sr sub(2)CaCu sub(2)O sub(8+ delta ). Our results reconcile the large anisotropy of Delta in k space (i.e. not a simple s wave) with the weak sensitivity of T sub(c) to defects and structural inhomogeneities (i.e. probably not a d sub(x(2)-y(2)) wave) and are compatible with the anisotropic s wave symmetry of Delta (k) in hole-doped high-T sub(c) superconductors. Experiments on electron irradiation of the twin-free single crystal of YBa sub(2)Cu sub(3)O sub(7- delta ) are consistent with a value of Chi approximately 0.3.