New millisecond pulsars ( MSPs ) in compact binaries provide a good opportunity to search for the most massive neutron stars . Their main-sequence companion stars are often strongly irradiated by the pulsar , displacing the effective center of light from their barycenter and making mass measurements uncertain . We present a series of optical spectroscopic and photometric observations of PSR J2215+5135 , a “ redback ” binary MSP in a 4.14 hr orbit , and measure a drastic temperature contrast between the dark/cold ( T _ { \mathrm { N } } =5660 ^ { +260 } _ { -380 } K ) and bright/hot ( T _ { \mathrm { D } } =8080 ^ { +470 } _ { -280 } K ) sides of the companion star . We find that the radial velocities depend systematically on the atmospheric absorption lines used to measure them . Namely , the semi-amplitude of the radial velocity curve of J2215 measured with magnesium triplet lines is systematically higher than that measured with hydrogen Balmer lines , by 10 % . We interpret this as a consequence of strong irradiation , whereby metallic lines dominate the dark side of the companion ( which moves faster ) and Balmer lines trace its bright ( slower ) side . Further , using a physical model of an irradiated star to fit simultaneously the two-species radial velocity curves and the three-band light curves , we find a center-of-mass velocity of K _ { 2 } =412.3 \pm 5.0 km s ^ { -1 } and an orbital inclination i=63.9 ^ { \circ } ^ { +2.4 } _ { -2.7 } . Our model is able to reproduce the observed fluxes and velocities without invoking irradiation by an extended source . We measure masses of M _ { 1 } =2.27 ^ { +0.17 } _ { -0.15 } M _ { \odot } and M _ { 2 } =0.33 ^ { +0.03 } _ { -0.02 } M _ { \odot } for the neutron star and the companion star , respectively . If confirmed , such a massive pulsar would rule out some of the proposed equations of state for the neutron star interior .