  It was recently demonstrated that the connectivities of bands emerging from
zero frequency in dielectric photonic crystals are distinct from their
electronic counterparts with the same space groups. We discover that, in an
AB-layer-stacked photonic crystal composed of anisotropic dielectrics, the
unique photonic band connectivity leads to a new kind of symmetry-enforced
triply degenerate points at the nexuses of two nodal rings and a Kramers-like
nodal line. The emergence and intersection of the line nodes are guaranteed by
a generalized 1/4-period screw rotation symmetry of Maxwell's equations. The
bands with a constant $k_z$ and iso-frequency surfaces near a nexus point both
disperse as a spin-1 Dirac-like cone, giving rise to exotic transport features
of light at the nexus point. We show that the spin-1 conical diffraction occurs
at the nexus point which can be used to manipulate the charges of optical
vortices. Our work reveals that Maxwell's equations can have hidden symmetries
induced by the fractional periodicity of the material tensor components and
hence paves the way to finding novel topological nodal structures unique to
photonic systems.
