A number of other FS CMa group objects show radial velocity (RV) or brightness variations possibly due to orbital motion, while other signs of binarity have been detected via spectroastrometry (e.g., ). Examples of the absorption-line spectra of cool components are shown in Figure 6. The exception is MWC 623 with both components of a visual brightness nearly the same and the only binary system in the FS CMa group that was known from the time of discovery of the B phenomenon. The strengths of these lines suggest the presence of cool components, which are typically Δ V ≥ 2 mag fainter than the hot components. In addition to the aforementioned defining features of the group objects, absorption lines of neutral metals have been detected in the spectra of some of them. The newly found objects were described in. With the group having been defined, several dozen new members and candidates have been found in various catalogs, such as the IRAS catalog, the NOMAD catalog, and others. The diversity of the secondary components in FS CMa objects is discussed in the context of non-conservative binary evolution. Nevertheless, we detected regular radial velocity variations of the spectral lines in MWC 728, 3 Pup, and AS 386 and we found absorption lines typical of cool stars in the spectra of MWC 645, AS 174, and several other objects. It is hard to detect the secondary components in these systems because of veiling and distortions by the circumstellar matter because of the relative faintness of the companions. We explore the hypothesis that the gaseous-and-dusty envelopes of FS CMa objects are due to either earlier or ongoing mass transfer between the binary system components. These properties are hard to explain in the framework of the evolution of single stars with luminosities between ~300 and ~30,000 L ⊙ typical of most B-type stars. The objects exhibit strong emission-line spectra with both permitted and forbidden lines suggesting the presence of a B-type star as well as strong IR excesses due to radiation of circumstellar dust. This paper describes recent studies of the FS CMa-type objects, a group of stars showing the B phenomenon defined in 2007.
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