  We have observed with XMM-Newton four radiatively efficient active type 1
galaxies with black hole masses < 10^6 Msun, selected optically from the SDSS.
We show here that their soft X-ray spectrum exhibits a soft excess with the
same characteristics as that observed ubiquitously in radio-quiet Seyfert 1
galaxies and type 1 quasars, both in terms of temperatures and strength.
However, even when the soft X-ray excess is modelled with a pure thermal disc,
its luminosity turns out to be much lower than that expected from accretion
theory for the given temperature, casting further doubts on a thermal
interpretation for soft excesses. While alternative scenarios for the nature of
the soft excess (namely smeared ionized absorption and disc reflection) cannot
be distinguished on a pure statistical basis, we point out that the absorption
model produces a strong correlation between absorbing column density and
ionization state, which may be difficult to interpret and is most likely
spurious. As for reflection, it does only invoke standard ingredients of any
accretion model for radiatively efficient sources such as a hard X-rays source
and a relatively cold (though partially ionized) accretion disc, and therefore
seems the natural choice to explain the soft excess in most (if not all) cases.
The reflection model is also consistent with the additional presence of a
thermal disc component with the theoretically expected temperature (although,
again, with smaller-than-expected total luminosity). The observed active
galaxies are among the most variable in X-rays and their excess variance is
among the largest. This is in line with their relatively small black hole mass
and with expectations from simple power spectra models. (abridged)
