  The Praesepe open cluster represents a puzzle since it has about the same age
as the Hyades, and only slightly different metallicity, yet previous ROSAT
observations resulted in a detection rate of cluster sources significantly
lower than for the Hyades. We present a new 50 ksec observation of Praesepe
performed with the EPIC instrument on board XMM-Newton, which resulted in the
detection of ~ 200 sources, including 48 cluster members. We detected all
solar-type (F-G) stars in the field of view, 90% of the K stars and 70% of the
M stars. We find that the distribution of X-ray luminosities of solar-type
Praesepe members is comparable to that of the Hyades, in contrast with the
previous ROSAT results; however, the disagreement between the ROSAT and
XMM-Newton results appears to be mostly due to X-ray faint Praesepe members
falling outside the XMM-Newton field of view, while it is considerably reduced
when considering only the subsample of stars in the ROSAT survey in common with
the present observation. The finding supports an earlier suggestion that
Praesepe may be formed by two merged clusters of different age.
