  We used the 50 cm Binocular Network (50BiN) telescope at Delingha Station
(Qinghai Province) of Purple Mountain Observatory (Chinese Academy of Sciences)
to obtain simultaneous $V$- and $R$-band observations of the old open cluster
NGC 188. Our aim was a search for populations of variable stars. We derived
light-curve solutions for six W Ursae Majoris (W UMa) eclipsing-binary systems
and estimated their orbital parameters. The resulting distance to the W UMas is
independent of the physical characteristics of the host cluster. We next
determined the current best period--luminosity relations for contact binaries
(CBs; scatter $\sigma < 0.10$ mag). We conclude that CBs can be used as
distance tracers with better than 5\% uncertainty. We apply our new relations
to the 102 CBs in the Large Magellanic Cloud, which yields a distance modulus
of $(m-M_V)_0=18.41\pm0.20$ mag.
