  AIMS. While observational evidence shows that most of the decline in a star's
X-ray activity occurs between the age of the Hyades (~8 x 10^8 yrs) and that of
the Sun, very little is known about the evolution of stellar activity between
these ages. To gain information on the typical level of coronal activity at a
star's intermediate age, we studied the X-ray emission from stars in the 1.9
Gyr old open cluster NGC 752. METHODS. We analysed a ~140 ks Chandra
observation of NGC 752 and a ~50 ks XMM-Newton observation of the same cluster.
We detected 262 X-ray sources in the Chandra data and 145 sources in the
XMM-Newton observation. Around 90% of the catalogued cluster members within
Chandra's field-of-view are detected in the X-ray. The X-ray luminosity of all
observed cluster members (28 stars) and of 11 cluster member candidates was
derived. RESULTS. Our data indicate that, at an age of 1.9 Gyr, the typical
X-ray luminosity of the cluster members with M=0.8-1.2 Msun is Lx = 1.3 x 10^28
erg s^-1, so approximately a factor of 6 less intense than that observed in the
younger Hyades. Given that Lx is proportional to the square of a star's
rotational rate, the median Lx of NGC 752 is consistent, for t > 1 Gyr, to a
decaying rate in rotational velocities v_rot ~ t^-alpha with alpha ~ 0.75,
steeper than the Skumanich relation (alpha ~ 0.5) and significantly steeper
than observed between the Pleiades and the Hyades (where alpha < 0.3),
suggesting that a change in the rotational regimes of the stellar interiors is
taking place at t ~ 1 Gyr.
