Radial velocities of pulsating subdwarf B stars: KPD 2109+4401 and PB 8783

High-speed spectroscopy of two pulsating subdwarf B stars, KPD 2109+4401 an d PB 8783, is presented. Radial motions are detected with the same frequenc ies as reported from photometric observations and with amplitudes of simila r to2 km s(-1) in two or more independent modes. These represent the first direct observations of surface motion arising from multimode non-radial osc illations in subdwarf B stars. In the case of the sdB+F binary PB 8783, the velocities of both components are resolved; high-frequency oscillations ar e found only in the sdB star and not the F star. There also appears to be e vidence for mutual motion of the binary components. If confirmed, it implie s that the F-type companion is greater than or similar to1.2 times more mas sive than the sdB star, while the amplitude of the F-star acceleration over 4 h would constrain the orbital period to lie between 0.5 and 3.2 d.