A mystery solved: the mass ratio of the dwarf nova EM Cygni

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 do nor star). The K2-5V star contributes approximately 16 per cent of the ligh t from the system and if not taken into account has a considerable effect u pon radial velocity measurements of the mass donor star. We obtain a new ra dial velocity amplitude for the mass donor star of K-2 = 202 +/- 3 km s(-1) , compared with the value of K-2 = 135 +/- 3 km s(-1) obtained in Stover, R obinson & Nather's classic study of EM Cyg. The revised value of the amplit ude, combined with a measurement of rotational broadening of the mass donor , v sin i = 140 +/- 6 km s(-1), leads to a new mass ratio of q = M-2/M-1 = 0.88 +/- 0.05. This solves a long-standing problem with EM Cyg, because Sto ver et al.'s measurements indicated a mass ratio q > 1, a value that should have led to dynamically unstable mass transfer for the secondary mass dedu ced 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 M. and 1.12 +/- 0.08 M. for the mass donor and white dwarf respectively. T he mass donor is evolved, because it has a later spectral type (K3) than it s 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.