Difference frequencies of nonradial pulsation modes and repetitive   outbursts in classical Be stars
  Space photometry has revealed rich pulsation spectra in classical Be stars. Often frequency pairs can be found that differ by nearly the frequency of a third genuine stellar variability. The lowest currently known of these so-called difference (or $\Delta$) frequencies reach 0.03 c/d, which is the upper limit of outburst frequencies. The potential of such slow variations for pulsation-assisted mass loss is discussed. 
D. Baade, Th. Rivinius, A. Pigulski, A. Carciofi, and BEST
astro-ph.SR