  Only a small number of high mass stars (> 30 Mo) have fundamental parameters
(i.e. masses and radii) measured with high enough accuracy from eclipsing
binaries to constrain formation and evolutionary models of massive stars. This
work aims to increase this limited sample, by studying the 4 massive eclipsing
binary candidates discovered by Bonanos in the young massive cluster Westerlund
1. We present new follow-up echelle spectroscopy of these binaries and models
of their light and radial velocity curves. We obtain fundamental parameters
(i.e. masses, radii) for the 8 component stars, finding masses that span a
range of 10-40 Mo, and contributing accurate fundamental parameters for 1
additional very massive star, the 33 Mo component of W13. WR77o is found to
have a ~40 Mo companion, which provides a second dynamical constraint on the
mass of the progenitor of the magnetar known in the cluster. We also use W13 to
estimate the first, direct, eclipsing binary distance to Westerlund 1 and
therefore the magnetar, and find it to be at 3.7 +/- 0.6 kpc. Our results
confirm previous evidence for a high mass for the progenitor of the magnetar.
In addition, the availability of eclipsing binaries with accurate parameters
opens the way for direct, independent, high precision eclipsing binary distance
measurements to Westerlund 1.
