We present the results of a speckle interferometric survey for close visual companions, mainly among 29 of the apparently brightest Wolf-Rayet (W-R) stars. Only one target, WR 48 \ h Mus, was resolved as a close astrometric binary (with a separation of 46 ^9 mas). This system is probably a triple comprising a short-period W-R binary plus a distant O supergiant companion. Although our binary detection fraction is low, it is not an unexpected result given the selection e †ects that militate against easy detection of binaries. New, higher resolution observations will almost certainly increase the fraction of binaries. There are four known binaries among the six W-R stars in our sample that have nonthermal radio emission, and this connection supports the idea that the nonthermal emission originates in the wind-wind collision between components.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

The {\it Microvariability and Oscillations of Stars (MOST)} satellite observed the B supergiant HD 163899 (B2 Ib/II) for 37 days as a guide star and detected 48 frequencies $\la$ 2.8 c d$^{-1}$ with amplitudes of a few milli-magnitudes (mmag) and less. The frequency range embraces g- and p-mode pulsations. It was generally thought that no g-modes are excited in less luminous B supergiants because strong radiative damping is expected in the core. Our theoretical models, however, show that such g-modes are excited in massive post-main-sequence stars, in accordance with these observations. The nonradial pulsations excited in models between $20M_\odot$ at $\log T_{\rm eff} \approx 4.41$ and $15M_\odot$ at $\log T_{\rm eff} \approx 4.36$ are roughly consistent with the observed frequency range. Excitation by the Fe-bump in opacity is possible because g-modes can be partially reflected at a convective zone associated with the hydrogen-burning shell, which significantly reduces radiative damping in the core. The {\it MOST} light curve of HD 163899 shows that such a reflection of g-modes actually occurs, and reveals the existence of a previously unrecognized type of variable, slowly pulsating B supergiants (SPBsg) distinct from $\alpha$ Cyg variables. Such g-modes have great potential for asteroseismology.Comment: 24 pages, 9 figures, Astrophysical Journal in pres