3U 0900-40

S. Rappaport and J. McClintock, Massachusetts Institute of Technology, report that the SAS-3 Group has measured the Doppler shift in the 282s.9 x-ray pulse period of 3U 0900-40 = Vel X-1 (IAUC 2794). Observations were made between June 18.8 and 24.2 and between July 19.3 and 25.3 UT. The Doppler shifts are in phase with the 8.96-day orbital period of the compact object around the B0.5Ib star HD 77581. The best-fit parameters (and 90-percent confidence error limits) for the binary system are: P = 282s.8913 +/- 0s.0004 (heliocentric); a sin i = 109 +/- 4 light seconds; K_x = 268 +/- 12 km/s; f(M) = 17.3 +/- 2.0 M_sol; e = 0.15 +/- 0.05; w = 157o +/- 24o; geometric center of eclipse = 1975 July 18.6 +/- 0.2 UT. Emersion from eclipse was observed on July 19.3, but the preceding orbital elements, combined with the eclipse duration (1.9 days) determined from Uhuru data (Forman et al. 1973, Astrophys. J. 182, L103), predict emersion at least 0.1 day later than this time. The determination of the semimajor axis (hence the mass function) is fairly insensitive to the choice of eccentricity. These measurements, when coupled with the minimum likely orbital velocity (K_opt = 23 km/s) inferred for HD 77581 (Avni and Bahcall 1975, Astrophys. J. Lett. in press), yield a lower limit for the mass of the x-ray star of m_x >= 1.7 M_sol. The existence of a high 'Q' periodicity rules out the possibility that Vel X-1 is a black hole, and the mass makes it unlikely that it is a white dwarf.