  We have searched for Extremely Red Objects (EROs) around faint mid-IR
selected galaxies in ELAIS fields. We find a significant overdensity, by
factors of 2 to 5, of these EROs compared to field EROs in the same region and
literature random field ERO counts. The excess is similar to that found
previously in the fields of known high redshift quasars and AGN. While with the
present data it cannot be definitely shown whether the overdensity is
physically connected to the mid-IR source, a derived radial distribution does
suggest this. The fraction of EROs among K-selected galaxies is high in the
mid-IR fields in agreement with the picture that the EROs responsible for the
overdensity are members of high redshift clusters of galaxies. We find R-K>5
selected EROs to be more clustered around the mid-IR galaxies than I-K>4 EROs,
though statistics are weak because of small numbers. However, this would be
consistent with a cluster/galaxy group scenario if, as we argue, the former
colour selection finds preferentially more strongly clustered early type
galaxies, whereas the latter selection includes a larger fraction of dusty
EROs. Finally, using the mid-IR data, we are able to limit the fraction of
ULIRG type very dusty EROs at K<18 magnitude to less than 10% of the total ERO
population.

Astrophysics