  In the cores of ellipticals, clusters, and groups of galaxies, the gas has a
cooling time shorter than 1 Gyr. It is possible to probe cooling flows through
the detection of Fe XVII and O VII emission lines, but so far O VII has not
been detected in any individual object. The Reflection Grating Spectrometers
(RGS) aboard XMM-Newton are currently the only instruments able to detect O VII
in extended objects such as elliptical galaxies and galaxy clusters. We
searched for evidence of O VII through all the archival RGS observations of
galaxy clusters, groups of galaxies, and elliptical galaxies focusing on those
with core temperatures below 1 keV. We have discovered O VII resonance (21.6A)
and forbidden (22.1A) lines for the first time in the spectra of individual
objects. O VII was detected at a level higher than three sigma in six
elliptical galaxies: M 84, M 86, M 89, NGC 1316, NGC 4636, and NGC 5846. M 84,
M 86 and M 89 are members of the Virgo Cluster, the others are central dominant
galaxies of groups, and most them show evidence of O VI in UV spectra. We
detect no significant trend between the Fe XVII and O VII
resonance-to-forbidden line ratios, possibly because of the limited statistics.
The observed line ratios <Fe(r/f), O(r/f)>= (0.52+/-0.02, 0.9+/-0.2) indicate
that the spectra of all these ellipticals are affected by resonance scattering,
suggesting low turbulence. Deeper exposures will help to understand whether the
Fe XVII and O VII lines are both produced by the same cooling gas or by
multiphase gas. Our O VII luminosities correspond to 0.2-2 Msun/yr, which agree
with the predictions for ellipticals. Such weak cooling rates would not be
detected in clusters because their spectra are dominated by the emission of
hotter gas, and owing to their greater distance, the expected O VII line flux
would be undetectable.
