1BRESSI 2011 uses the method of DYLLA 1973 but finds serious errors in that experiment that greatly reduce its accuracy. The BRESSI 2011 limit assumes that ${{\mathit n}}$ $\rightarrow$ ${{\mathit p}}{{\mathit e}^{-}}{{\mathit \nu}_{{{e}}}}$ conserves charge. Thus the limit applies equally to the charge of the neutron.
2SENGUPTA 2000 uses the difference between the observed rate of of rotational energy loss by the binary pulsar PSR B1913+16 and the rate predicted by general relativity to set this limit. See the paper for assumptions.