  We report spectroscopic observations of the 2.63 day, detached, F-type
main-sequence eclipsing binary V2154 Cyg. We use our observations together with
existing $uvby$ photometric measurements to derive accurate absolute masses and
radii for the stars good to better than 1.5%. We obtain masses of M1 = 1.269
+/- 0.017 M(Sun) and M2 = 0.7542 +/- 0.0059 M(Sun), radii of R1 = 1.477 +/-
0.012 R(Sun) and R2 = 0.7232 +/- 0.0091 R(Sun), and effective temperatures of
6770 +/- 150 K and 5020 +/- 150 K for the primary and secondary stars,
respectively. Both components appear to have their rotations synchronized with
the motion in the circular orbit. A comparison of the properties of the primary
with current stellar evolution models gives good agreement for a metallicity of
[Fe/H] = -0.17, which is consistent with photometric estimates, and an age of
about 2.2 Gyr. On the other hand, the K2 secondary is larger than predicted for
its mass by about 4%. Similar discrepancies are known to exist for other cool
stars, and are generally ascribed to stellar activity. The system is in fact an
X-ray source, and we argue that the main site of the activity is the secondary
star. Indirect estimates give a strength of about 1 kG for the surface magnetic
field on that star. A previously known close visual companion to V2154 Cyg is
shown to be physically bound, making the system a hierarchical triple.
