  We present the detection of a low surface-brightness stellar counterpart to
an enormous (190 kpc) ring of neutral hydrogen (HI) gas that surrounds the
nearby radio galaxy B2 0648+27. This system is currently in an evolutionary
stage between major merger and (radio-loud) early-type galaxy. In a previous
paper we investigated in detail the timescales between merger, starburst and
AGN activity in B2 0648+27, based on its unusual multi-wavelength properties
(large-scale HI ring, dominating post-starburst stellar population and
infra-red luminosity). In this Research Note we present deep optical B- and
V-band imaging that provides further evidence for the merger origin of B2
0648+27. The host galaxy shows a distorted optical morphology and a broad tidal
arm is clearly present. A low surface-brightness stellar tail or partial ring
curls around more than half the host galaxy at a distance of up to 55 kpc from
the centre of the galaxy, following the large-scale, ring-like HI structure
that we detected previously around this system. The gas and stars in this ring
are most likely tidally expelled material that slowly fell back onto the host
galaxy after the merger event. There also appear to be sites of star formation
in the HI ring that may have formed within the gaseous tidal debris after the
merger. We argue that the observed properties of the gas and stars in B2
0648+27, as well as the apparent time-delay between the merger and the
starburst event, may be the logical result of a merger between two gas-rich
disk galaxies with a prominent bulge, or of a merger between an elliptical and
a gas-rich spiral galaxy. There also appears to be a significant time-delay
between the merger/starburst event and the current episode of radio-AGN
activity.
