  We investigate multicolour imaging data of a complete sample of 19 low
redshift (z<0.2) quasar host galaxies. The sample was imaged in four optical
(BVRi) and three near-infrared bands (JHK_s). Galaxy types, structural
parameters and robust host galaxy luminosities are extracted for all bands by
means of two-dimensional deblending of galaxy and nucleus. For the disc
dominated fraction of host galaxies (Sa and later) the optical and
optical-to-NIR colours agree well with the average colours of inactive galaxies
of same type. The bulge dominated galaxies (E/S0) on the other hand appear a
significant 0.35 mag bluer in (V-K) than their inactive counterparts, being as
blue as the discs in the sample. This trend is confirmed by fitting population
synthesis models to the extracted broad band SEDs: the stellar population age
of the bulge dominated hosts lies around a few Gyr, much younger than expected
for old evolved ellipticals. Comparison to other studies suggests a strong
trend for stellar age in elliptical host galaxies with luminosity.
Intermediately luminous elliptical hosts have comparably young populations,
either intrinsically or from an enhanced star formation rate potentially due to
interaction, the most luminous and massive ellipticals on the contrary show old
populations. The correspondence between the nuclear activity and the blue
colours suggests a connection between galaxy interaction, induced star
formation and the triggering of nuclear activity. However, the existence of
very symmetric and undisturbed disks and elliptical host galaxies emphasised
that other mechanisms like minor merging or gas accretion must exist.
