  The production of virtual photons is a very sensitive probe of the properties
of the quark-gluon plasma. As they are experimentally detected by lepton pairs,
they suffer from a large background arising from hadron decays. Light-flavour
hadrons dominate at low invariant masses below $m_{ee}\sim0.5$ GeV and heavy
flavours above. These contributions must therefore also be taken into account
in experimental analyses at the LHC. In this paper, we calculate the direct
contribution from virtual photons produced in the Drell-Yan process with an
additional jet in POWHEG and find that it is significant at low invariant
masses. We also simulate the background contributions from $c\bar c$ and $b
\bar b$ production with POWHEG and quantify the theoretical uncertainties due
to variations of the perturbative scales and parton distribution functions. We
find larger relative and absolute uncertainties for the lighter $c$ quarks than
for heavier $b$ quarks.
