  We present photometric analysis and follow-up spectroscopy for a population
of extremely red stellar objects extracted from the point-source catalogue of
the INT Photometric H-Alpha Survey (IPHAS) of the northern galactic plane. The
vast majority of these objects have no previous identification. Analysis of
optical, near- and mid-infrared photometry reveals that they are mostly
highly-reddened asymptotic giant branch stars, with significant levels of
circumstellar material. We show that the distribution of these objects traces
galactic extinction, their highly reddened colours being a product of both
interstellar and circumstellar reddening. This is the first time that such a
large sample of evolved low-mass stars has been detected in the visual and
allows optical counterparts to be associated with sources from recent infrared
surveys.
  Follow-up spectroscopy on some of the most interesting objects in the sample
has found significant numbers of S-type stars which can be clearly separated
from oxygen-rich objects in the IPHAS colour-colour diagram. We show that this
is due to the positions of different molecular bands relative to the
narrow-band H-alpha filter used for IPHAS observations. The IPHAS (r' -
H-alpha) colour offers a valuable diagnostic for identifying S-type stars. A
selection method for identifying S-type stars in the galactic plane is briefly
discussed and we estimate that over a thousand new objects of this type may be
discovered, potentially doubling the number of known objects in this short but
important evolutionary phase.
