  We analyze extensive BVRI photometry and radial velocity measurements for
three double-lined deeply-eclipsing binary stars in the field of the old open
cluster NGC 7142. The short period (P = 1.9096825 d) detached binary V375 Cep
is a high probability cluster member, and has a total eclipse of the secondary
star. The characteristics of the primary star (M=1.288+/-0.017 Sun) at the
cluster turnoff indicate an age of 3.6 Gyr (with a random uncertainty of 0.25
Gyr), consistent with earlier analysis of the color-magnitude diagram. The
secondary star (M = 0.871+/-0.008 Msun) is not expected to have evolved
significantly, but its radius is more than 10% larger than predicted by models.
Because this binary system has a known age, it is useful for testing the idea
that radius inflation can occur in short period binaries for stars with
significant convective envelopes due to the inhibition of energy transport by
magnetic fields. The brighter star in the binary also produces a precision
estimate of the distance modulus, independent of reddening estimates:
(m-M)_V=12.86+/-0.07.
  The other two eclipsing binary systems are not cluster members, although one
of the systems (V2) could only be conclusively ruled out as a present or former
member once the stellar characteristics were determined. That binary is within
0.5 degrees of edge-on, is in a fairly long-period eccentric binary, and
contains two almost indistiguishable stars. The other binary (V1) has a small
but nonzero eccentricity (e=0.038) in spite of having an orbital period under 5
d.
