  Herschel surveys have found large numbers of sources with red far-IR colours,
and spectral energy distributions (SEDs) rising from 250 to 500$\mu$m: 500
risers. The nature and role of these sources is not fully understood. We here
present Submillimeter Array (SMA) interferometric imaging at 200 GHz of a
complete sample of five 500 risers with F500 $>$ 44 mJy selected within a 4.5
square degree region of the XMMLSS field. These observations can resolve the
separate components of multiple sources and allow cross identification at other
wavelengths using the extensive optical-to-IR data in this field. Of our five
targets we find that two are likely gravitationally lensed, two are multiple
sources, and one an isolated single source. Photometric redshifts, using
optical-to-IR data and far-IR/submm data, suggest they lie at redshifts $z \sim
2.5 - 3.5$. Star formation rates and stellar masses estimated from the SEDs
show that the majority of our sources lie on the star-formation rate-stellar
mass `main sequence', though with outliers both above and below this relation.
Of particular interest is our most multiple source which consists of three
submm emitters and one submm-undetected optical companion within a 7 arcsecond
region, all with photometric redshifts $\sim$ 3. One of the submm emitters in
this group lies above the `main sequence', while the optical companion lies
well below the relation, and has an estimated stellar mass of 3.3$\pm 1.3
\times$10$^{11}$ M$_{\odot}$. We suggest this object is a forming brightest
cluster galaxy (BCG) in the process of accreting actively star forming
companions.
