Physical parameter study of eight W Ursae Majoris-type contact binaries
  in NGC 188
  We used the newly commissioned 50 cm Binocular Network (50BiN) telescope at
Qinghai Station of Purple Mountain Observatory (Chinese Academy of Sciences) to
observe the old open cluster NGC 188 in V and R as part of a search for
variable objects. Our time-series data span a total of 36 days. Radial velocity
and proper-motion selection resulted in a sample of 532 genuine cluster
members. Isochrone fitting was applied to the cleaned cluster sequence,
yielding a distance modulus of (m - M)0V = 11.35 \pm 0.10 mag and a total
foreground reddening of E(V - R) = 0.062 \pm 0.002 mag. Light-curve solutions
were obtained for eight W Ursae Majoris eclipsing-binary systems (W UMas) and
their orbital parameters were estimated. Using the latter parameters, we
estimate a distance to the W UMas which is independent of the host cluster's
physical properties. Based on combined fits to six of the W UMas (EP Cep, EQ
Cep, ES Cep, V369 Cep, and--for the first time--V370 Cep and V782 Cep), we
obtain an average distance modulus of (m - M)0V = 11.31 \pm 0.08 mag, which is
comparable with that resulting from our isochrone fits. These six W UMas
exhibit an obvious period-luminosity relation. We derive more accurate physical
parameters for the W UMa systems and discuss their initial masses and ages. The
former show that these W UMa systems have likely undergone angular-momentum
evolution within a convective envelope (W-type evolution). The ages of the W
UMa systems agree well with the cluster's age.
