  R Coronae Borealis stars (RCB) are a rare type of evolved carbon-rich
supergiant stars that are increasingly thought to result from the merger of two
white dwarfs, called the Double degenerate scenario. This scenario is also
studied as a source, at higher mass, of type Ia Supernovae (SnIa) explosions.
Therefore a better understanding of RCBs composition would help to constrain
simulations of such events. We searched for and studied RCB stars in the EROS
Magellanic Clouds database. We also extended our research to DY Per type stars
(DYPers) that are expected to be cooler RCBs (T~3500 K) and much more numerous
than their hotter counterparts. The light curves of ~70 millions stars have
been analysed to search for the main signature of RCBs and DYPers: a large drop
in luminosity. Follow-up optical spectroscopy was used to confirm each
photometric candidate found. We have discovered and confirmed 6 new Magellanic
Cloud RCB stars and 7 new DYPers, but also listed new candidates: 3 RCBs and 14
DYPers. We estimated a range of Magellanic RCB shell temperatures between 360
and 600 K. We confirm the wide range of absolute luminosity known for RCB
stars, M_V~-5.2 to -2.6. Our study further shows that mid-infrared surveys are
ideal to search for RCB stars, since they have thinner and cooler circumstellar
shells than classical post-AGB stars. In addition, by increasing the number of
known DYPers by ~400%, we have been able to shed light on the similarities in
the spectral energy distribution between DYPers and ordinary carbon stars. We
also observed that DYPer circumstellar shells are fainter and hotter than those
of RCBs. This suggests that DYPers may simply be ordinary carbon stars with
ejection events, but more abundance analysis is necessary to give a status on a
possible evolutionnary connexion between RCBs and DYPers.
