We present high and intermediate resolution spectroscopy of the X-ray nova Centaurus X-4 during its quiescent phase .
Our analysis of the absorption features supports a { K } 3 - { K } 5 V spectral classification for the companion star , which contributes \simeq 75 per cent of the total flux at H \alpha .
Using the high resolution spectra we have measured the secondary star ’ s rotational broadening to be v \sin i = 43 \pm 6 km s ^ { -1 } and determined a binary mass ratio of q = 0.17 \pm 0.06 .
Combining our results for K _ { 2 } and q with the published limits for the binary inclination , we constrain the mass of the compact object and the secondary star to the ranges 0.49 < M _ { 1 } < 2.49 M _ { \odot } and 0.04 < M _ { 2 } < 0.58 M _ { \odot } .
A Doppler image of the H \alpha line shows emission coming from the secondary star , but no hotspot is present .
We discuss the possible origins of this emission .