  We present the rest-frame optical spectral energy distribution and stellar
masses of six Herschel- selected gravitationally lensed dusty, star-forming
galaxies (DSFGs) at 1 < z < 3. These galaxies were first identified with
Herschel/SPIRE imaging data from the Herschel Astrophysical Terahertz Large
Area Survey (H-ATLAS) and the Herschel Multi-tiered Extragalactic Survey
(HerMES). The targets were observed with Spitzer/IRAC at 3.6 and 4.5um. Due to
the spatial resolution of the IRAC observations at the level of 2 arcseconds,
the lensing features of a background DSFG in the near-infrared are blended with
the flux from the foreground lensing galaxy in the IRAC imaging data. We make
use of higher resolution Hubble/WFC3 or Keck/NIRC2 Adaptive Optics imaging data
to fit light profiles of the foreground lensing galaxy (or galaxies) as a way
to model the foreground components, in order to successfully disentangle the
foreground lens and background source flux densities in the IRAC images. The
flux density measurements at 3.6 and 4.5um, once combined with Hubble/WFC3 and
Keck/NIRC2 data, provide important constraints on the rest-frame optical
spectral energy distribution of the Herschel-selected lensed DSFGs. We model
the combined UV- to millimeter-wavelength SEDs to establish the stellar mass,
dust mass, star-formation rate, visual extinction, and other parameters for
each of these Herschel-selected DSFGs. These systems have inferred stellar
masses in the range 8 x 10^10 to 4 x 10^11 Msun and star-formation rates of
around 100 Msun yr-1. This puts these lensed sub-millimeter systems well above
the SFR-M* relation observed for normal star-forming galaxies at similar
redshifts. The high values of SFR inferred for these systems are consistent
with a major merger-driven scenario for star formation.
