  We present an improved dynamical model of the X-ray binary and microquasar
XTE J1550-564 based on new moderate-resolution optical spectroscopy and
near-infrared photometry. By combining our new radial velocity measurements
with previous measurements obtained 2001 May at the 8.2m VLT and with light
curves, we find an orbital period of P=1.5420333 +/- 0.0000024 days and a
radial velocity semiamplitude of K_2=363.14 +/- 5.97$ km/sec, which together
imply an optical mass function of f(M)=7.65 +/- 0.38 solar masses. We find that
the projected rotational velocity of the secondary star is 55 +/- 5 km/sec,
which implies a very extreme mass ratio of Q=M/M_2=30. Using a model of a Roche
lobe-filling star and an azimuthally symmetric accretion disk, we fit
simultaneously optical light curves from 2001, near-infrared light curves from
2008 and all of the radial velocity measurements to derive system parameters.
We find an inclination of 74.7 +/- 3.8 deg and component masses of M_2=0.30 +/-
0.07 solar masses and M=9.10 +/- 0.61 solar masses for the secondary star and
black hole, respectively. The radius of the secondary star for the adopted
model is 1.75 +/- 0.12 solar radii. Using this radius, the average K_S
magnitude, and an extinction of A_K=0.507 +/- 0.050 mag, we find a distance of
4.38^{+0.58}_{-0.41} kpc, which is in good agreement with a recent distance
estimate based on HI absorption lines (abstract shortened).
