  The mass, origin and evolutionary stage of the binary system LB-1 has been
the subject of intense debate, following the claim that it hosts an
$\sim$70$M_{\odot}$ black hole, in stark contrast with the expectations for
stellar remnants in the Milky Way. We conducted a high-resolution,
phase-resolved spectroscopic study of the near-infrared Paschen lines in this
system, using the 3.5-m telescope at Calar Alto Observatory. We find that
Pa$\beta$ and Pa$\gamma$ (after proper subtraction of the stellar absorption
component) are well fitted with a standard double-peaked model, typical of disk
emission. We measured the velocity shifts of the red and blue peaks at 28
orbital phases: the line center has an orbital motion in perfect antiphase with
the stellar motion, and the radial velocity amplitude ranges from 8 to 13 km/s
for different choices of lines and profile modelling. We interpret this curve
as proof that the disk is tracing the orbital motion of the primary, ruling out
the circumbinary disk and the hierarchical triple scenarios. The phase-averaged
peak-to-peak half-separation (proxy for the projected rotational velocity of
the outer disk) is $\sim$70 km s$^{-1}$, larger than the stellar orbital
velocity and also inconsistent with a circumbinary disk. From those results, we
infer a primary mass 4--8 times higher than the secondary mass. Moreover, we
show that the ratio of the blue and red peaks (V/R intensity ratio) has a
sinusoidal behaviour in phase with the secondary star, which can be interpreted
as the effect of external irradiation by the secondary star on the outer disk.
Finally, we briefly discuss our findings in the context of alternative
scenarios recently proposed for LB-1. Definitive tests between alternative
solutions will require further astrometric data from $Gaia$.
