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The {\it Microvariability and Oscillations of Stars (MOST)} satelliteobserved the B supergiant HD 163899 (B2 Ib/II) for 37 days as a guide star anddetected 48 frequencies $\la$ 2.8 c d$^{-1}$ with amplitudes of a fewmilli-magnitudes (mmag) and less. The frequency range embraces g- and p-modepulsations. It was generally thought that no g-modes are excited in lessluminous B supergiants because strong radiative damping is expected in thecore. Our theoretical models, however, show that such g-modes are excited inmassive post-main-sequence stars, in accordance with these observations. Thenonradial 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 roughlyconsistent with the observed frequency range. Excitation by the Fe-bump inopacity is possible because g-modes can be partially reflected at a convectivezone associated with the hydrogen-burning shell, which significantly reducesradiative damping in the core. The {\it MOST} light curve of HD 163899 showsthat such a reflection of g-modes actually occurs, and reveals the existence ofa previously unrecognized type of variable, slowly pulsating B supergiants(SPBsg) distinct from $\alpha$ Cyg variables. Such g-modes have great potentialfor asteroseismology.Comment: 24 pages, 9 figures, Astrophysical Journal in pres

MOSTDetectsg‐ andp‐Modes in the B Supergiant HD 163899 (B2 Ib/II)