  We have monitored the benchmark L dwarf binary Kelu-1AB over the past 3 years
to derive improved spectral types and luminosities for the individual
components. The astrometric measurements enable us to compute the orbital
parameters and result in the first dynamical mass estimate for the system. We
obtained resolved high angular resolution, near-IR images with HST and the
VLT/NACO adaptive optics instrument in the J, H and Ks bands. In addition we
used NACO to achieve the first spatially resolved mid-resolution spectra in the
H- and K-band for a precise spectral type determination. The near-IR spectrum
of Kelu-1A reveals a distinct dip in the H-band providing evidence that Kelu-1
A itself is a spectroscopic binary. We derive spectral types of L0.5+- 0.5 for
Kelu- 1 Aa and T7.5+-1 for Kelu-1 Ab. Kelu-1B is classified as spectral type
L3pec +-1.5. From the relative orbit, we derive an orbital period of 38 +8 -6
years and a semi-major axis of 6.4+2.4 -1.3 AU. This yields the first
dynamically determined total system mass of 177 +113 -55 Mjup for the Kelu-1AB
system, with the uncertainties mainly attributed to the error of the system
distance measurement and the yet missing apastron passage. The derived
properties of Kelu-1AB allow us to test common theoretical models. The
comparison of our results with color-magnitude diagrams based on evolutionary
models yields a slightly revised age estimate (0.3 - 0.5 Gyr) and a discrepancy
between dynamically and theoretically derived masses, stressing the importance
for further dynamical mass determinations of brown dwarf binaries.
