its merger history. As such, getting a grip on the shape of the halo offers a potential constraint on the dark matter model [@Davis1985A].

The shape of dark matter haloes can be classified by the shape parameter $q$, using the ratios between the vertical axis $c$ and radial axis $a$, such that $q = c/a$. This divides the potential halo shapes up into three classes: prolate ($q > 1$), oblate ($q < 1$) and spherical ($q \sim 1$). This is, of course, only a simplified version of reality, where we can also expect triaxial shapes and changes of shape with radius and history [@VeraCiro2014]. However, for haloes with masses ${\lower.5ex\hbox{$\; \buildrel > \over \sim \;$}}10^{12.3} h^{-1} M_\odot$ the haloes can be adequately described by a single vertical-to-radial axis ratio [@Schneider2012A].

Direct observations of the shape of haloes is tricky, as there are only few tracers that offer a clear view on the vertical gravitational potential of the halo. The flat rotation curves of galaxies, while an excellent tracer of the radial potential of the halo, provides no information in the vertical direction. Luckily, some tracers do exist. The stellar streams of stripped, in-falling galaxies can be used to measure the 