  Ultralight scalars, if they exist as theorized, could form clouds around
rapidly rotating black holes. Such clouds are expected to emit continuous,
quasimonochromatic gravitational waves that could be detected by LIGO and
Virgo. Here we present results of a directed search for such signals from the
Cygnus X-1 binary, using data from Advanced LIGO's second observing run. We
find no evidence of gravitational waves in the 250-750 Hz band. Without
incorporating existing measurements of the Cygnus X-1 black hole spin, our
results disfavor boson masses in $6.4 \leq \mu/(10^{-13} {\rm eV}) \leq 8.0$,
assuming that the black hole was born $5 \times 10^6$ years ago with a
nearly-extremal spin. We then focus on a string axiverse scenario, in which
self-interactions enable a cloud for high black-hole spins consistent with
measurements for Cygnus X-1. In that model, we constrain the boson masses in
$9.6 \leq \mu/(10^{-13} {\rm eV}) \leq 15.5$ for a decay constant $f_a\sim
10^{15}$ GeV. Future application of our methods to other sources will yield
improved constraints.
