  Randich and Schmitt [1995, A&A 298, 115] found that the coronal activity of
solar-type and low mass stars in Praesepe is significantly lower than that of
stars in the Hyades cluster. We have carried out several tests in order to find
a possible explanation for this result. We have measured radial velocities of
two groups of Praesepe stars (a dF-dK sample and a dM sample) and have measured
H$\alpha$ as a chromospheric activity index for the dM sample. We conclude that
the Praesepe catalog used in the X-ray analysis does not contain a significant
number of non-members. The comparison of the H$\alpha$ equivalent widths for
the M dwarfs in Praesepe with those in the Hyades indicates that, at least for
stars in this mass range, the Praesepe stars are as active or more active than
their Hyades counterparts. We have also analyzed a few ROSAT PSPC pointings of
Praesepe in order to obtain a new and independent estimate of the X-ray
luminosities and upper limits for a small sample of Praesepe members concluding
that the small differences between the old and new upper limits are not large
enough to explain the dichotomy in the X-ray properties of Praesepe and the
Hyades. Therefore, our examination of the available data does not provide a
clear reason to explain why the X-ray luminosity functions of the two clusters
are different. Part of the explanation could be found in the binaries.
Speculatively, these clusters could have different orbital period
distributions, with more short period binaries among the Hyades, which would
show larger coronal activity.
