A mystery solved : the mass ratio of the dwarf nova

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 kms −1 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 v sini = 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 leads to masses of 0.99±0.12M⊙ and 1.12±0.08M⊙ 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.