  We study discrete symmetries satisfied by helical $p$-wave superconductors
with d-vectors $k_{x}\hat{x}\pm k_{y}\hat{y}$ or $k_{y}\hat{x}\pm k_{x}\hat{y}$
and transformations brought by the symmetry operations to ferromagnet and
spin-singlet superconductors, which show intimate associations with transport
properties in heterojunctions including helical superconductor. Especially, the
partial symmetries of the Hamiltonian under the spin-rotation and
gauge-rotation operations are responsible for novel invariances of the
conductance in tunnel junctions and new selection rules of the lowest current
and peculiar phase diagrams in Josephson junctions which are reported recently.
The symmetries of constructed free energies for Josephson junctions are also
analyzed which are consistent with the results from Hamiltonian.
