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We present a comparison of azimuthally averaged radial surface brightness μ( r ) profiles and analytical bulge–disc decompositions (de Vaucouleurs,  r  1/4  bulge plus exponential disc) for spiral galaxies using  Hubble Space Telescope /Advanced Camera for Surveys  V ‐band imaging from the Space Telescope A901/2 Galaxy Evolution Survey (STAGES). In the established classification scheme, antitruncated μ( r ) profiles (Type III) have a broken exponential disc with a shallower region beyond the break radius  r  brk . The excess light at large radii ( r  &gt;  r  brk ) can either be caused by an outer exponential disc (Type III‐d) or an extended spheroidal component (Type III‐s). Using our comparisons, we determine the contribution of bulge light at  r  &gt;  r  brk  for a large sample of 78 (barred/unbarred, Sa‐Sd) spiral galaxies with outer disc antitruncations (). In the majority of cases (∼85 per cent), evidence indicates that excess light at  r  &gt;  r  brk  is related to an outer shallow disc (Type III‐d). Here, the contribution of bulge light at  r  &gt;  r  brk  is either negligible (∼70 per cent) or too little to explain the antitruncation (∼15 per cent). However in the latter cases, bulge light can affect the measured disc properties (e.g. μ brk , outer scalelength). In the remaining cases (∼15 per cent), light at  r  &gt;  r  brk  is dominated by the bulge (Type III‐s). Here, for most cases the bulge profile dominates at all radii and only occasionally ( galaxies, ∼5 per cent) extends beyond that of a dominant disc and explains the excess light at  r  &gt;  r  brk . We thus conclude that in the vast majority of cases antitruncated outer discs cannot be explained by bulge light and thus remain a pure disc phenomenon.

Antitruncated stellar light profiles in the outer regions of STAGES spiral galaxies: bulge or disc related?