  We have discovered that the spectrum of the well-known dwarf nova EM Cyg is
contaminated by light from a K2-5V star (in addition to the K-type mass donor
star). The K2-5V star contributes approximately 16 per cent of the light from
the system and if not taken into account has a considerable effect upon radial
velocity measurements of the mass donor star. We obtain a new radial velocity
amplitude for the mass donor star of K2 = 202 +/- 3 km/s, which compares with
the value of K2 = 135 +/- 3 km/s obtained in Stover, Robinson & Nather's
classic 1981 study of EM Cyg. The revised value of the amplitude combined with
a measurement of rotational broadening of the mass donor vsini = 140 +/- 6
km/s, leads to a new mass ratio of q = M2/M1 = 0.88 +/- 0.05. This solves a
long standing problem with EM Cyg because Stover et al.'s measurements
indicated a mass ratio q > 1, a value which should have led to dynamically
unstable mass transfer for the secondary mass deduced by Stover et al. The
revised value of the mass ratio combined with the orbital inclination i = 67
+/- 2 degrees leads to masses of 0.99 +/- 0.12 Msun and 1.12 +/- 0.08 Msun for
the mass donor and white dwarf respectively. The mass donor is evolved, since
it has a later spectral type (K3) than its mass would imply. We discuss whether
the K star could be physically associated with EM Cyg or not, and present the
results of the spectroscopic study.
