Most massive neutron star ever detected used General Relativity effectto come up with mass

Massive neutron star is exactly that : Nature News
"The researchers used the National Radio Astronomy Observatory Green
Bank Telescope in West Virginia to examine a rotating neutron star,
called J1614-2230, which orbits a companion 'white dwarf' star as part
of a binary star system. The neutron star emits radio pulses at regular
millisecond intervals, but as each pulse passes by the white dwarf on
its way to Earth it is delayed in a way that depends on the mass of the
dwarf and varies as the stars orbit each other.

This 'Shapiro delay' is an effect of general relativity, which states
that the gravitational field of massive objects causes clocks to tick
more slowly and radio pulses to be retarded. By combining measurements
of the Shapiro delay, the period of the stars' orbit and the speed of
the neutron star, the scientists calculated the mass of the neutron star. "
http://www.nature.com/news/2010/1010....2010.565.html