centering some markup under a bracket

[Denis . Roegel]

Hi,

I am trying to center a markup under a bracket, for annotation
purposes. Unfortunately, my markup only appears left of the bracket:

\version "2.12.1"

\score{
  \new Staff <<
    \new Voice = "one" {
       \cadenzaOn

       a'\startGroup b'\stopGroup

    }
    \new Lyrics \lyricsto "one" { X }
  >>
  \layout {
     \context {
       \Staff \consists "Horizontal_bracket_engraver"
     }
   }
}

Is there any way to fix this? Thanks,

Denis Roegel