Spectroscopic observations of the soft X-ray transient 4U 1543-47 reveal a radial velocity curve with a period of P = 1.123 \pm 0.008 days and a semi-amplitude of K _ { 2 } = 124 \pm 4 km s ^ { -1 } .
The resulting mass function is f ( M ) = 0.22 \pm 0.02 M _ { \sun } .
We classify the secondary star as A2V , in agreement with previous work , and measure T _ { eff } = 9000 \pm 500 K and E ( B - V ) = 0.50 \pm 0.05 from fits to synthetic spectra .
We derive a distance of d = 9.1 \pm 1.1 kpc if the secondary is on the main sequence .
We see no emission lines from the accretion disk , and the measured fraction of disk light in the B and V bands is 10 % and 21 % respectively .
The V and I light curves exhibit two waves per orbital cycle with amplitudes \approx 0.08 mag .
We modeled the light curves as ellipsoidal variations in the secondary star and derive extreme inclination limits of 20 \leq i \leq 40 \arcdeg .
The formal 3 \sigma limits on the inclination from a simultaneous fit to the V and I light curves are 24 \leq i \leq 36 \arcdeg for a mass ratio Q \equiv M _ { 1 } / M _ { 2 } \geq 1 .
However , there are systematic effects in the data that the model does not account for , so the above constraints should be treated with caution .
We argue that the secondary star is still on the main sequence with the mass transfer being driven by expansion due to normal main sequence evolution .
If the secondary star has a mass near the main sequence values for early A-stars ( 2.3 \leq M _ { 2 } \leq 2.6 M _ { \sun } ) , then the best fits for the 3 \sigma inclination range ( 24 \leq i \leq 36 \arcdeg ) and the 3 \sigma mass function range ( 0.16 \leq f ( M ) \leq 0.28 M _ { \sun } ) imply a primary mass in the range 2.7 \leq M _ { 1 } \leq 7.5 M _ { \sun } .
The mass of the compact object in 4U 1543-47 is likely to be in excess of \approx 3 M _ { \sun } , which is widely regarded as the maximum mass of a stable neutron star .
Thus we conclude 4U 1543-47 most likely contains a black hole .