  Light scalar mesons can be understood as dynamically generated resonances.
They arise as 'companion poles' in the propagators of quark-antiquark seed
states when accounting for hadronic loop contributions to the self-energies of
the latter. Such a mechanism may explain the overpopulation in the scalar
sector - there exist more resonances with total spin $J=0$ than can be
described within a quark model.
  Along this line, we study an effective Lagrangian approach where the
isovector state $a_{0}(1450)$ couples via both non-derivative and derivative
interactions to pseudoscalar mesons. It is demonstrated that the propagator has
two poles: a companion pole corresponding to $a_{0}(980)$ and a pole of the
seed state $a_{0}(1450)$. The positions of these poles are in quantitative
agreement with experimental data. Besides that, we investigate similar models
for the isodoublet state $K_{0}^{\ast}(1430)$ by performing a fit to $\pi K$
phase shift data in the $I=1/2,$ $J=0$ channel. We show that, in order to fit
the data accurately, a companion pole for the $K_{0}^{\ast}(800)$, that is, the
light $\kappa$, is required. A large-$N_{c}$ study confirms that both
resonances below $1$ GeV are predominantly four-quark states, while the heavy
states are quarkonia.
