Rotation and magnetic activity are intimately linked in main-sequence stars of G or later spectral types. The presence and level of magnetic activity depend on stellar rotation, and rotation itself is strongly influenced by strength and topology of t
he magnetic fields. Open clusters represent especially useful targets to investigate the rotation/activity/age connection. The open cluster M11 has been studied as a part of the RACE-OC project (Rotation and ACtivity Evolution in Open Clusters), which is aimed at exploring the evolution of rotation and magnetic activity in the late-type members of open clusters with different ages. Photometric observations of the open cluster M11 were carried out in June 2004 using LOAO 1m telescope. The rotation periods of the cluster members are determined by Fourier analysis of photometric data time series. We further investigated the relations between the surface activity, characterized by the light curve amplitude, and rotation. We have discovered a total of 75 periodic variables in the M11 FoV, of which 38 are candidate cluster members. Specifically, among cluster members we discovered 6 early-type, 2 eclipsing binaries and 30 bona-fide single periodic late-type variables. Considering the rotation periods of 16 G-type members of the almost coeval 200-Myr M34 cluster, we could determine the rotation period distribution from a more numerous sample of 46 single G stars at an age of about 200-230 Myr and determine a median rotation period P=4.8d. A comparison with the younger M35 cluster (~150 Myr) and with the older M37 cluster (~550 Myr) shows that G stars rotate slower than younger M35 stars and faster than older M37 stars. The measured variation of the median rotation period is consistent with the scenario of rotational braking of main-sequence spotted stars as they age.
V-band time-series CCD photometric observations of the intermediate-age open cluster M11 were performed to search for variable stars. Using these time-series data, we carefully examined light variations of all stars in the observing field. A total of
82 variable stars were discovered, of which 39 stars had been detected recently by Hargis et al. (2005). On the basis of observational properties such as variable period, light curve shape, and position on a color-magnitude diagram, we classified their variable types as 11 delta Scuti-type pulsating stars, 2 gamma Doradus-type pulsating stars, 40 W UMa-type contact eclipsing binaries, 13 Algol-type detached eclipsing binaries, and 16 eclipsing binaries with long period. Cluster membership for each variable star was deduced from the previous proper motion results (McNamara et al. 1977) and position on the color-magnitude diagram. Many pulsating stars and eclipsing binaries in the region of M11 are probable members of the cluster.
