  Several rotational transitions of water have been identified toward evolved
stars in the ground vibrational state as well as in the first excited state of
the bending mode. In the latter vibrational state of water, the 658 GHz J =
1_1,0-1_0,1 rotational transition is often strong and seems to be widespread in
late-type stars. Our main goals are to better characterize the nature of the
658 GHz emission, compare the velocity extent of the 658 GHz emission with SiO
maser emission to help locate the water layers and, more generally, investigate
the physical conditions prevailing in the excited water layers of evolved
stars. Another goal is to identify new 658 GHz emission sources and contribute
in showing that this emission is widespread in evolved stars. Eleven evolved
stars were extracted from our mini-catalog of existing and potential 658 GHz
sources for observations with the APEX telescope equipped with the SEPIA
receiver. The 13CO J=6-5 line was placed in the same receiver sideband for
simultaneous observation with the 658 GHz line of water. We have compared the
658 GHz line properties with our H2O radiative transfer models in stars and we
have compared the velocity ranges of the 658 GHz and SiO J=2-1, v=1 maser
lines. All stars show 658 GHz emission with a peak flux density in the range
50-70 Jy to 2000-3000 Jy. We have shown that the 658 GHz line is masing and we
found that the 658 GHz velocity extent tends to be correlated with that of the
SiO maser suggesting that both emission lines are excited in circumstellar
layers close to the central star. Broad and stable line profiles are observed
at 658 GHz. This could indicate maser saturation although we have tentatively
provided first information on time variability at 658 GHz.
