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We present the analysis of the HI content of a sample of early-type galaxies(ETGs) in low-density environments (LDEs) using the data set provided by theArecibo Legacy Fast ALFA (ALFALFA) survey. We compare their properties to thesample in the Virgo cluster that we studied in a previous paper (di SeregoAlighieri et al. 2007, Paper I). We have selected a sample of 62 nearby ETGs(V< 3000 km/s) in an area of the sky where the ALFALFA data are alreadyavailable (8h<RA<16h, 4 deg<DEC<16deg), avoiding the region of the Virgocluster. Among these, 39 have absolute B magnitudes fainter than M_B = -17.Fifteen out of 62 galaxies have been firmly detected with ALFALFA (\sim 25%).Five additional galaxies show a weaker HI emission (S/N \sim 4) and they willneed deeper observations to be confirmed. All together, our analysis doublesthe number of known gas-rich ETGs in this area. The HI detection rate is 44% inluminous ETGs (M_B < -17) and 13% in dwarf ETGs (M_B > -17). In both cases itis 10 times higher than that of the Virgo cluster. The presence of gas can berelated to a recent star formation activity: 60% of all ETGs with HI haveoptical emission line ratios typical of star-forming galaxies and blue colourssuggesting the presence of young stellar populations, especially in the dwarfsubsample. We show that the HI detection rate of ETGs depends both on theenvironment and mass. The fraction of early-type systems with neutral hydrogenis higher in more massive objects when compared to early-type dwarfs. The ETGsin LDEs seem to have more heterogeneous properties than their Virgo clustercounterparts, since they are able to retain a cold interstellar gas componentand to support star formation activity even at recent epochs.

The HI content of early-type galaxies from the ALFALFA survey. II. The case of low density environments