  (abridged) We performed a homogeneous and systematic analysis of simultaneous
X-ray and optical/UV properties of a group of 15 gamma-ray loud AGN, using
observations performed with XMM-Newton. The sample is composed of 13 blazars (6
BL Lac and 7 Flat-Spectrum Radio Quasar) and 2 radio galaxies, that are
associated with detections at energies >100 MeV. The data for 7 of them are
analyzed here for the first time, including the first X-ray observation of PKS
1406-706. Then, the spectral characteristics of the sources in the present
sample have been compared with those in previous catalogs of blazars and other
AGN, in order to search for any difference or long term change. All the
selected sources appear to follow the classic "blazar sequence" and the
spectral energy distributions (SED) built with the present X-ray and optical/UV
data and completed with historical data, confirm the findings of previous
studies on this type of sources. Some sources display interesting features
worth noting: four of them, namely AO 0235+164, PKS 1127-145, S5 0836+710 and
PKS 1830-211 show the presence of an intervening absorption system along the
line of sight, but only the latter is known to be surely gravitationally
lensed. AO 0235+164 was detected during an outburst and its SED shows a clear
shift of the synchrotron peak. 3C 273 shows a change in the state with respect
to the previous BeppoSAX observations, that can be interpreted as an increase
of the Seyfert-like component and a corresponding decline of the jet emission.
This is consistent with the monitoring at radio wavelengths performed during
the same years. PKS 1406-706 is detected with a flux higher than in the past,
but with a corresponding low optical flux. Moreover, although it is classified
as FSRQ, the SED can be modelled with a simple synchrotron self-Compton model.
