[en] Compared with d-band metals of the same valency, La shows a similar anomalous thermal behaviour to that observed for Ce (e.g. low melting point, low Debye frequency). It is shown that the soft phonons in these systems are induced by the f-band, which is known to intersect the Fermi level in α-Ce and to lie slightly above it in La. The RPA dielectric constant is therefore calculated for a simple two-band model of d- and f-electrons and it is shown that it is enhanced due to the high density of states of the f-band near the Fermi energy. The phonon frequencies, renormalized by this enhanced dielectric constant, are thus lowered. (author)