Pulsars are magnetized rotating compact objects .
They spin down due to magnetic dipole radiation and wind emission .
If photon has a nonzero mass , the spin down rate would be smaller than the zero mass case .
We show that an upper limit of the photon mass , i.e . m _ { \gamma } \lesssim h / Pc ^ { 2 } , may be placed if a pulsar with period P is observed to spin down .
Recently , a white dwarf ( WD ) — M dwarf binary , AR Scorpii was discovered to emit pulsed broadband emission with pulses .
The spin-down luminosity of the WD can comfortably power the non-thermal radiation from the system .
Applying our results to the WD pulsar with P = 117 ~ { } s , we obtain a stringent upper limit of the photon mass between m _ { \gamma } < 6.3 \times 10 ^ { -50 } ~ { } g assuming a vacuum dipole spindown , and m _ { \gamma } < 9.6 \times 10 ^ { -50 } ~ { } g assuming a spindown due to a fully developed pulsar wind .