The long-term stability of oscillations during thermonuclear X-ray bursts: Constraining the binary X-ray mass function

We report on the long-term stability of the millisecond oscillations observed with the Rossi X-Ray Timing Explorer during thermonuclear X-ray bursts from the low-mass X-ray binaries (LMXBs) 4U 1728-34 and 4U 1636-53. We show that bursts from 4U 1728-34 spanning more than 1.5 yr have observed asymptotic oscillation periods that are within 0.2 mu s of each other, well within the magnitude that could be produced by the orbital motion of the neutron star in a canonical LMXB. This stability implies a timescale to change the oscillation period of more than 23,000 yr in this system and suggests a highly stable process, such as stellar rotation, as the mechanism producing the oscillations. For 4U 1636-53, which has an orbital period of 3.8 hr, we show that the offsets in the asymptotic oscillation periods from three different bursts can be consistently interpreted as due to the orbital velocity of the neutron star with v sin i/c approximate to 4.25 x 10(-4). An updated optical ephemeris for the epoch of maximum light from V801 Arae would provide a strong test of this interpretation. We discuss the constraints on the X-ray mass function, which in principle can be derived using this technique.