  Star-forming galaxies at redshifts z>6 are likely responsible for the
reionization of the universe, and it is important to study the nature of these
galaxies. We present three candidates for z~7 Lyman-break galaxies (LBGs) from
a 155 arcmin^2 area in the CANDELS/COSMOS field imaged by the deep FourStar
Galaxy Evolution (zFourGE) survey. The FourStar medium-band filters provide the
equivalent of R~10 spectroscopy, which cleanly distinguishes between z~7 LBGs
and brown dwarf stars. The distinction between stars and galaxies based on an
object's angular size can become unreliable even when using HST imaging; there
exists at least one very compact z~7 candidate (FWHM~0.5-1 kpc) that is
indistinguishable from a point source. The medium-band filters provide narrower
redshift distributions compared with broad-band-derived redshifts. The UV
luminosity function derived using the three z~7 candidates is consistent with
previous studies, suggesting an evolution at the bright end (MUV -21.6 mag)
from z~7 to z~5. Fitting the galaxies' spectral energy distributions, we
predict Lyman-alpha equivalent widths for the two brightest LBGs, and find that
the presence of a Lyman-alpha line affects the medium-band flux thereby
changing the constraints on stellar masses and UV spectral slopes. This
illustrates the limitations of deriving LBG properties using only broad-band
photometry. The derived specific star-formation rates for the bright LBGs are
~13 per Gyr, slightly higher than the lower-luminosity LBGs, implying that the
star-formation rate increases with stellar mass for these galaxies.
