  We present simultaneous NuSTAR and Swift observations of the black hole
transient IGR J17091-3642 during its 2016 outburst. By jointly fitting six
NuSTAR and four Swift spectra, we found that during this outburst the source
evolves from the hard to the hard/soft intermediate and back to the hard state,
similar to the 2011 outburst. Unlike in the previous outburst, in this case we
observed both a broad emission and an moderately broad absorption line in our
observations. Our fits favour an accretion disc with an inclination angle of
$\sim$$45\deg$ with respect to the line of sight and a high iron abundance of
$3.5\pm0.3$ in units of the solar abundance. We also observed heartbeat
variability in one NuSTAR observation. We fitted the phase-resolved spectra of
this observation and found that the reflected emission varies independently
from the direct emission, whereas in the fits to the average spectra these two
quantities are strongly correlated. Assuming that in IGR J17091-3642 the inner
radius of the disc both in the average and the phase-resolved spectra is
located at the radius of the innermost stable circular orbit, with 90%
confidence the spin parameter of the black hole in this system is $-0.13\leq
a_{*}\leq0.27$.
