  A class of singlet superconductors with a gap function $\Delta({\bf k},
\omega_n)$ which is {\it odd} in both momentum and Matsubara frequency was
proposed recently \cite{ba}. To show an instability in the {\it odd} gap
channel, a model phonon propagator was used with the $p$-wave interaction
strength larger than the $s$-wave. We argue that the positive scattering matrix
element entering the Eliashberg equations leads to a constraint on the relative
strength of $p$- and $s$-wave interactions which inhibits odd pairing. However,
a general spin dependent electron-electron interaction can satisfy all
constraints and produce the odd singlet gap. A possibility which may lead to an
odd gap is a strongly antiferromagnetically correlated system, such as a
high-$T_c$ material.
