  We present optical and near-IR data of three Type II supernovae (SNe II), SN
2008bm, SN 2009aj, and SN 2009au. These SNe display the following common
characteristics: signs of early interaction of the ejecta with circumstellar
material (CSM), blue $B\!-\!V$ colours, weakness of metal lines, low expansion
velocities, and $V$-band absolute magnitudes 2-3 mag brighter than those
expected for normal SNe II based on their expansion velocities. Two more SNe
reported in the literature (SN 1983K and LSQ13fn) share properties similar to
our sample. Analysing this set of five SNe II, which are luminous for their low
expansion velocities (LLEV), we find that their properties can be reproduced
assuming ejecta-CSM interaction that lasts between 4-11 weeks post explosion.
The contribution of this interaction to the radiation field seems to be the
dominant component determining the observed weakness of metal lines in the
spectra rather than the progenitor metallicity. Based on hydrodynamic
simulations, we find that the interaction of the ejecta with a CSM of ~3.6
M$_\odot$ can reproduce the light curves and expansion velocities of SN 2009aj.
Using data collected by the Chilean Automatic Supernova Search, we estimate an
upper limit for the LLEV SNe II fraction to be 2-4 per cent of all normal SNe
II. With the current data-set, it is not clear whether the LLEV events are a
separated class of SNe II with a different progenitor system, or whether they
are the extreme of a continuum mediated by CSM interaction with the rest of the
normal SN II population.

High Energy Astrophysical Phenomena