  FS CMa stars are low-luminosity objects showing the B[e] phenomenon whose
evolutionary state remains a puzzle. These stars are surrounded by compact
disks of warm dust of unknown origin. Hitherto, membership of FS CMa stars to
coeval populations has never been confirmed. The discovery of low-luminosity
line emitters in the young massive clusters Mercer 20 and Mercer 70 prompts us
to investigate the nature of such objects. We intend to confirm membership to
coeval populations in order to characterize these emission-line stars through
the cluster properties. Based on ISAAC/VLT medium-resolution spectroscopy and
NICMOS/HST photometry of massive cluster members, new characterizations of
Mercer 20 and Mercer 70 are performed. Coevality of each cluster and membership
of the newly-discovered B[e] objects are investigated using our observations as
well as literature data of the surroundings. Infrared excess and narrow-band
photometric properties of the B[e] stars are also studied. We confirm and
classify 22 new cluster members, including Wolf-Rayet stars and blue
hypergiants. Spectral types (O9-B1.5 V) and radial velocities of B[e] objects
are compatible with the remaining cluster members, while emission features of
Mg II, Fe II], and [Fe II] are identified in their spectra. The ages of these
stars are 4.5 and 6 Myr, and they show mild infrared excesses. We confirm the
presence of FS CMa stars in the coeval populations of Mercer 20 and Mercer 70.
We discuss the nature and evolutionary state of FS CMa stars, discarding a
post-AGB nature and introducing a new hypothesis about mergers. A new search
method for FS CMa candidates in young massive clusters based on narrow-band
Paschen-alpha photometry is proposed and tested in photometric data of other
clusters, yielding three new candidates.
