  We present a detailed study of NGC 6791, NGC 6811, NGC 6819 and NGC 6866, the
four open clusters that are located in the Kepler prime field. We use new CCD
UBV(RI)KC photometry, which was combined with Gaia EDR3 photometric/astrometric
data, to derive the astrophysical parameters with two independent methods - one
of them the fitCMD algorithm. Furthermore, we provide among others estimates of
the mass and mass function, the cluster structure, derive the cluster orbits,
and discuss the cluster dynamics. All objects belong to the older open cluster
population (1-7Gyr), are in an advanced dynamical stage with signs of mass
segregation, and are located close to the solar circle, but show a large range
in respect of radii, member stars or observed cluster mass (100-2000 Msolar).
For the three younger objects we were also able to provide photometric
metallicity estimates, which confirms their status as clusters with a roughly
solar metallicity. The most outstanding object is clearly NGC 6791, a very old
cluster with a high metallicity at a distance of about 4.5 kpc from the Sun. We
estimate a probable radial migration by about 7 kpc, resulting in a birth
position close to the Galactic center.
