  The importance of the contributions of the longitudinally polarized virtual
photon in ep collisions is investigated. We derive the factorization formulae
for the unpolarized inclusive and semi-inclusive ep collisions in an arbitrary
reference frame. The numerical calculations for the prompt photons production
in the unpolarized Compton process (e p --> e gamma X) at the ep HERA collider
are performed in the Born approximation. We studied various distributions in
the ep centre-of-mass frame and found that the differential cross section for
the longitudinally polarized intermediate photon and the term due to the
interference between the longitudinal- and transverse- polarization states of
the photon are small, i.e. below 10% of the cross section. Moreover, these two
contributions almost cancel one another, leading to a stronger domination of
the transversely polarized virtual photon, even for its large virtuality.
Relevance of the resolved longitudinal photon in a jet production in DIS events
at HERA is commented. A relatively large (about 30%) effect due to the
longitudinal-transverse interference term was found in the azimuthal-angle
distribution in the Breit frame.
