Context :

Aims : We present a study of the stellar populations of a sample of 39 local , field early-type galaxies whose H i properties are known from interferometric data .
Our aim is to understand whether stellar age and chemical composition depend on the H i 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 .

Methods : 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 .

Results : We do not find any trend in the stellar populations parameters with M ( H i ) .
However , we do find that , at stellar velocity dispersion \sigma \leq 230 km/s , 2/3 of the galaxies with less than 10 ^ { 8 } M _ { \odot } of H i are centrally rejuvenated , while none of the H i -richer systems is .
Furthermore , none of the more massive , \sigma \geq 230 km/s-objects is centrally rejuvenated independently on their H i 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 H i mass is necessary ( but not sufficient ) for a galaxy to host bright ionised-gas emission .

Conclusions : A plausible interpretation of our results is that gas-rich mergers play a significant role in E/S0 formation , especially at lower \sigma .
Within this picture , H i -poor , centrally-rejuvenated objects could form in mergers where gas angular-momentum removal ( and therefore inflow ) is efficient ; H i -rich galaxies with no significant age gradients ( but possibly uniformly young ) could be formed in interactions characterised by high-angular momentum gas .