Optical photometry and spectroscopy of the accretion-powered millisecond pulsar HETE J1900.1-2455

We present phase resolved optical photometry and spectroscopy of the accreting millisecond pulsar HETE J1900.1-2455. Our R-band light curves exhibit a sinusoidal modulation, at close to the orbital period, which we initially attributed to X-ray heating of the irradiated face of the secondary star. However, further analysis reveals that the source of the modulation is more likely due to superhumps caused by a precessing accretion disc. Doppler tomography of a broad Ha emission line reveals an emission ring, consistent with that expected from an accretion disc. Using the velocity of the emission ring as an estimate for the projected outer disc velocity, we constrain the maximum projected velocity of the secondary to be 200 km s(-1), placing a lower limit of 0.05 M-circle dot on the secondary mass. For a 1.4 M-circle dot primary, this implies that the orbital inclination is low, less than or similar to 20 degrees. Utilizing the observed relationship between the secondary mass and the orbital period in short-period cataclysmic variables, we estimate the secondary mass to be similar to 0.085 M-circle dot, which implies an upper limit of similar to 2.4 M-circle dot for the primary mass.