  Using high spatial resolution Hubble Space Telescope/Advanced Camera for
Surveys archival imaging observations of Arp 116, centred on the elliptical
galaxy NGC 4649, we explore the novel technique of pixel-by-pixel analysis of
the galaxy's colour-magnitude diagramme to search for any evidence of recent
enhanced star formation due to the apparent tidal interaction with its spiral
companion, NGC 4647. From a detailed analysis of the system's geometry, and
based on additional circumstantial evidence from extant multi-wavelength
observations, we conclude that, while there may be grounds for the
tidal-interaction assumption for this system, any interaction has thus far been
of insufficient strength to trigger an enhanced level of recent star formation
in the elliptical component, although close inspection of our colour images
shows a faint excess of bluer pixels (a ~0.20 mag bluer "loop") in the
elliptical galaxy on the side of the spiral companion. Given that there appears
to be a moderate reservoir of available gas for ongoing star formation
(although at low column density), this suggests that we are currently
witnessing the onset of the tidal interaction between NGC 4647 and NGC 4649. In
addition, the triggering of new star formation in NGC 4649 may be significantly
impeded due to the much lower mass of the spiral component.
