  \textit{Superradiance} can trigger the formation of an ultra-light boson
cloud around a spinning black hole. Once formed, the boson cloud is expected to
emit a nearly periodic, long-duration, gravitational-wave signal. For boson
masses in the range $(10^{-13}-10^{-11})$ eV, and stellar mass black holes,
such signals are potentially detectable by gravitational wave detectors, like
Advanced LIGO and Virgo. In this {\it Letter} we present full band upper limits
for a generic all-sky search for periodic gravitational waves in LIGO O2 data,
and use them to derive - for the first time - direct constraints on the
ultra-light scalar boson field mass.
