  We present a study of the stellar populations of a sample of 39 local, field
early-type galaxies whose HI properties are known from interferometric data.
Our aim is to understand whether stellar age and chemical composition depend on
the HI content of galaxies. As a by-product of our analysis, we also study
their ionised gas content and how it relates to the neutral hydrogen gas.
Stellar populations and ionised gas are studied from optical long-slit spectra.
We determine stellar age, metallicity and alpha-to-iron ratio by analysing a
set of Lick/IDS line-strength indices measured from the spectra after modelling
and subtracting the ionised-gas emission. We do not find any trend in the
stellar populations parameters with M(HI). However, we do find that, at stellar
velocity dispersion below 230 km/s, 2/3 of the galaxies with less than 100
million solar masses of HI are centrally rejuvenated, while none of the
HI-richer systems are. Furthermore, none of the more massive
(velocity-dispersion>230 km/s) objects are centrally rejuvenated independently
on their HI mass. Concerning the ionised gas, we detect emission in 60% of the
sample. This is generally extended and always carachterised by LINER-like
emission-line ratios at any radius. We find that a large HI mass is necessary
(but not sufficient) for a galaxy to host bright ionised-gas emission. A
plausible interpretation of our results is that gas-rich mergers play a
significant role in E/S0 formation, especially at lower mass. Within this
picture, HI-poor, centrally-rejuvenated objects could form in mergers where gas
angular-momentum removal (and therefore inflow) is efficient; HI-rich galaxies
with no significant age gradients (but possibly uniformly young) could be
formed in interactions characterised by high-angular momentum gas.
