  We compute the effects due to the virtual exchange (or the soft emission) of
a scalar particle with generic couplings to the top quark in $t\bar t$ pair
production at the LHC. We apply the results to two cases of interest, extending
and completing previous studies. First, we consider the indirect search for
light ($m_S<2 m_t$) top-philic scalars with CP-even and/or CP-odd interactions.
Second, we investigate how to set constraints on anomalous Yukawa couplings of
the Higgs boson to the top quark. Our results show that the current precision
of experimental data together with the accuracy of the SM predictions make such
indirect determinations a powerful probe for new physics.
