PSR J1738+0333 is one of the four millisecond pulsars known to be orbited by a white dwarf companion bright enough for optical spectroscopy .
Of these , it has the shortest orbital period , making it especially interesting for a range of astrophysical and gravity related questions .
We present a spectroscopic and photometric study of the white dwarf companion and infer its radial velocity curve , effective temperature , surface gravity and luminosity .
We find that the white dwarf has properties consistent with those of low-mass white dwarfs with thick hydrogen envelopes , and use the corresponding mass-radius relation to infer its mass ; M _ { WD } = 0.181 ^ { +0.007 } _ { -0.005 } M _ { \odot } .
Combined with the mass ratio q = 8.1 \pm 0.2 inferred from the radial velocities and the precise pulsar timing ephemeris , the neutron star mass is constrained to M _ { PSR } = 1.47 ^ { +0.07 } _ { -0.06 } M _ { \odot } .
Contrary to expectations , the latter is only slightly above the Chandrasekhar limit .
We find that , even if the birth mass of the neutron star was only 1.20 M _ { \odot } , more than 60 % of the matter that left the surface of the white dwarf progenitor escaped the system .
The accurate determination of the component masses transforms this system in a laboratory for fundamental physics by constraining the orbital decay predicted by general relativity .
Currently , the agreement is within 1 \sigma of the observed decay .
Further radio timing observations will allow precise tests of white dwarf models , assuming the validity of general relativity .