We demonstrate that rotational superradiance can be efficient in millisecond pulsars .
Measurements from the two fastest known pulsars PSR J1748-2446ad and PSR B1937+21 can place bounds on bosons with masses below 10 ^ { -11 } ~ { } \text { eV } .
The bounds are maximally good at masses corresponding to the rotation rate of the star , where scalar interactions that mediate forces \sim 10 ^ { 6 } times weaker than gravity are ruled out , exceeding existing fifth force constraints by 3 orders of magnitude .
For certain neutron star equations of state , these measurements would also constrain the QCD axion with masses between 5 \times 10 ^ { -13 } and 3 \times 10 ^ { -12 } ~ { } \text { eV } .
Despite the ability of most neutron star equations of state to support frequencies as high as \sim 1500 ~ { } \text { Hz } , the observed absence of pulsars above \sim 700 ~ { } \text { Hz } could be due to the existence of a new particle of mass \sim 2 \pi \times 1500 - 3000 ~ { } \text { Hz } or \sim 10 ^ { -11 } eV with a Yukawa coupling to nucleons .