  We propose a theoretical model of quasi-one-dimensional superconductors, with
attractive electron-electron interactions dominant in the singlet d-wave
channel and sub-dominant in the p-wave channel. We discuss, in the mean field
approximation, the effect of a magnetic field applied perpendicularly to the
direction of the lowest conductivity. The lowest free energy phase corresponds
to a singlet d-wave symmetry in low fields, but to a triplet symmetry in high
fields. A first order singlet-triplet phase transition is expected at moderate
applied fields of a few teslas. We propose to ascribe the recent critical field
and NMR experimental data, observed in superconducting (TMTSF)2ClO4 to such an
effect.
