Prompted by peculiar spectroscopic variability observed in SDSS/APOGEE H -band spectra , we monitored the Be star HD 55606 using optical spectroscopy and found that it is an exotic double-lined spectroscopic binary ( SB2 ) consisting of a Be star and a hot , compact companion that is probably an OB subdwarf ( sdOB ) star .
Motion of the sdOB star is traced by its impact on the strong He i lines , observed as radial velocity ( V _ { r } ) variable , double-peaked emission profiles with narrow central absorption cores .
Weak He ii 4686 Å absorption associated with the companion star is detected in most spectra .
Use of the emission peaks of low-ionization emission lines to trace the Be star V _ { r } and the He i lines to trace the companion star V _ { r } yields a circular orbital solution with a 93.8-day period and masses of M _ { Be } = 6.2 M _ { \odot } and M _ { sdOB } = 0.9 M _ { \odot } in the case of i = 80 ^ { \circ } .
HD 55606 exhibits a variety of phase-locked variability , including the development of shell lines twice per orbit .
The shell phases coincide with variation in the double emission peak separations , and both forms of variability are likely caused by a two-armed spiral density perturbation in the Be disk .
The intensity ratios of the double emission peaks are also phase-locked , possibly indicating heating by the sdOB star of the side of the Be disk facing it .
HD 55606 is a new member of the growing sample of Be+sdOB binaries , in which the Be star ’ s rapid rotation and ability to form a disk can be attributed to past mass transfer .