A new radial velocity study of the well-known dwarf nova SS Cygni: a low-mass binary system

ABSTRACT We present new high-dispersion spectroscopic observations of the cataclysmic variable SS Cygni at the end of an outburst (decline) and during the first days at quiescence. We determined the orbital parameters of the white dwarf (WD) and the red dwarf (RD) companion at quiescence. The mass functions at this stage are $M_{wd} \sin ^3{i} = 0.24 \pm 0.01$ M$_{\odot }$ and $M_{rd} \sin ^3{i} = 0.11 \pm 0.01$ M$_{\odot }$, and the separation is $a \sin i = 1.25 \pm 0.01$ R$_{\odot }$. Since this binary is not an eclipsing system, a direct value of the inclination i cannot be determined. However, published photometric curves and their model fitting yield the range $45^\circ \le i \le 56^\circ$. Using this interval we obtain masses of the WD and RD within the ranges $0.42\,\,\mathrm{M}_\circ \le M_{wd} \le 0.67$ M$_{\odot}$ and $0.19 \le M_{rd} \le 0.30$ M$_{\odot }$, respectively, and separation in the range $1.51 \le a \le 1.77$ R$_{\odot }$. This is an important result, as our range of masses is lower than those previously published. We argue that our determination of the semi-amplitude $K_{rd} = 158.6 \pm 0.7$ km s$^{-1}$ is a solid one. However, this is not the case of $K_{wd}$. The large scatter found in this parameter and the uncertainty in the inclination angle explain the large range in the masses published in previous works. Finally, we present Doppler tomography of several Balmer lines. We find that during the decline, the accretion disc is barely visible, showing mainly a hot spot, while during the minimum state a disc with a spiral-arm-like structure is observed.