  Rotation and solar-type magnetic activity are closely related to each other
in stars of G or later spectral types. Presence and level of magnetic activity
depend on star's rotation and rotation itself is strongly influenced by
strength and topology of the magnetic fields. Open clusters represent
especially useful targets to investigate the connection between rotation and
activity. The open cluster NGC2099 has been studied as a part of the RACE-OC
project, which is aimed at exploring the evolution of rotation and magnetic
activity in the late-type members of open clusters of different ages. Time
series CCD photometric observations of this cluster were collected during
January 2004. The relations between activity manifestations, such as the light
curve amplitude, and global stellar parameters are investigated. We have
discovered 135 periodic variables, 122 of which are candidate cluster members.
Determination of rotation periods of G- and K-type stars has allowed us to
better explore evolution of angular momentum at an age of about 500 Myr. A
comparison with the older Hyades cluster (~625 Myr) shows that the newly
determined distribution of rotation periods is consistent with the scenario of
rotational braking of main-sequence spotted stars as they age. However, a
comparison with the younger M34 cluster (~200 Myr) shows that the G8-K5 members
of these clusters have the same rotation period distribution, that is G8-K5
members in NGC2099 seem to have experienced no significant braking in the age
range from ~200 to ~500 Myr. Finally, NGC2099 members have a level of
photospheric magnetic activity, as measured by light curve amplitude, smaller
than in younger stars of same mass and rotation, suggesting that the activity
level also depends on some other age-dependent parameters.
