  Very young open clusters are ideal places to study the X-ray properties of a
homogeneous population of early-type stars. In this respect, the IC1805 open
cluster is very interesting as it hosts the O4If$^+$ star HD15570 thought to be
in an evolutionary stage intermediate between a normal O-star and a Wolf-Rayet
star. Such a star could provide a test for theoretical models aiming at
explaining the empirical scaling relation between the X-ray and bolometric
luminosities of O-type stars. We have observed IC1805 with XMM-Newton and
further collected optical spectroscopy of some of the O-star members of the
cluster. The optical spectra allow us to revisit the orbital solutions of
BD+60$^{\circ}$ 497 and HD15558, and provide the first evidence of binarity for
BD+60$^{\circ}$ 498. X-ray emission from colliding winds does not appear to
play an important role among the O-stars of IC1805. Notably, the X-ray fluxes
do not vary significantly between archival X-ray observations and our
XMM-Newton pointing. The very fast rotator BD+60$^{\circ}$ 513, and to a lesser
extent the O4If$^+$ star HD15570 appear somewhat underluminous. Whilst the
underluminosity of HD15570 is only marginally significant, its amplitude is
found to be compatible with theoretical expectations based on its stellar and
wind properties. A number of other X-ray sources are detected in the field, and
the brightest objects, many of which are likely low-mass pre-main sequence
stars, are analyzed in detail.
