  A combined effect of the orthorhombic crystal field and potential impurity
scattering on several superconducting states of a tetragonal symmetry is
studied within a weak-coupling mean field approach. It is shown that the
nonmagnetic impurities stabilize the states belonging to the identity
irreducible representation. The electronic specific heat jump at the phase
transition is analyzed. Its dependence on the potential scattering rate for
large impurity concentration is shown to be remarkably different for the states
with a nonzero value of the Fermi surface averaged order parameter than for
those with a vanishing one. In particular, very distinct signals from
d_{x^2-y^2} state in YBCO and d_{xy} state in BSCCO compound are predicted.
This effect may be used as a test for the presence of these states in the above
cuprates.
