Context :

Aims : We present the analysis of the H i content of a sample of early-type galaxies ( ETGs ) in low-density environments ( LDEs ) using the data set provided by the Arecibo Legacy Fast ALFA ( ALFALFA ) survey .
We compare their properties to the sample in the Virgo cluster that we studied in a previous paper ( di Serego Alighieri et al .
2007 , Paper I ) .
Our aim is both to investigate how the cool interstellar medium ( ISM ) of these systems depends on the galaxy mass and the environment and to relate the properties of the neutral hydrogen to the warm phases of the ISM .

Methods : We have selected a sample of 62 nearby ETGs ( V < 3000 km s ^ { -1 } ) in an area of the sky where the ALFALFA data are already available ( 8 ^ { h } < RA < 16 ^ { h } , 4 ^ { \circ } < DEC < 16 ^ { \circ } ) , avoiding the region of the Virgo cluster .
Among these , 39 have absolute B magnitudes fainter than M _ { B } = -17 .

Results : Fifteen out of 62 galaxies have been firmly detected with ALFALFA ( \sim 25 % ) .
Five additional galaxies show a weaker H i emission ( S/N \sim 4 ) and they will need deeper observations to be confirmed .
Eight objects had 21-cm measurements reported in the literature .
One by one comparison with the available material confirms , as expected , that ALFALFA data are , with rare exceptions , of equal or better quality than the best spectra previously obtained for these objects .
All together , our analysis doubles the number of known gas-rich ETGs in this area .
The H i detection rate is 44 % in luminous ETGs ( M _ { B } < -17 ) and 13 % in dwarf ETGs ( M _ { B } > -17 ) .
In both cases it is 10 times higher than that of the Virgo cluster .
The presence of gas can be related to a recent star formation activity : 60 % of all ETGs with H i have optical emission line ratios typical of star-forming galaxies and blue colours suggesting the presence of young stellar populations , especially in the dwarf subsample .

Conclusions : We show that the H i detection rate of ETGs depends both on the environment and mass .
The fraction of early-type systems with neutral hydrogen is higher in more massive objects when compared to early-type dwarfs .
The ETGs in LDEs seem to have more heterogeneous properties than their Virgo cluster counterparts , since they are able to retain a cold interstellar gas component and to support star formation activity even at recent epochs .